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This page contains important dates or occurrences during the events of Percy Jackson and the Olympians and The Heroes of Olympus series. Rick Riordan has stated that there is no specific year or timeline that the series takes place as book time is not tied to real time. 
- January 2011 - The events of The Sword of Summer and The Hidden Oracle occur.
- January 13, 1995 - Magnus Chase is born to Natalie Chase and Frey.
- Nico di Angelo's birthday (while undecided) is stated by Rick Riordan to be January 28th, 1924, (this would mean that his mother's death was around 1934). 
- April 2010 - Percy Jackson awakes after months of deep sleep induced by Hera/Juno, and begins training with Lupa at the Wolf House.
- May 2006 - Yancy Academy visits the Manhattan Museum of Greek and Roman artifacts.
- May 2010 - The satyr Gleeson Hedge marries the aura Mellie.
- June 11, 2006 - Percy Jackson, Annabeth Chase, and Grover Underwood begin their quest for the Master Bolt.
- June 14, 2006-The quest for the Master Bolt continues.
- June 15, 2006 - Percy, Annabeth, and Grover arrive at the Lotus Hotel and Casino. They do not leave until June 20, five days later.
- June 18, 2007 - Percy, Annabeth, Grover, Tyson, and Clarisse La Rue arrive in Miami, Florida after their odyssey in the Sea of Monsters.
- After hearing Clarisse's prophecy, Percy sends her back to Camp Half-Blood with the Golden Fleece.
- Luke Castellan, Agrius, and Oreius forcibly bring Percy, Annabeth, Grover, and Tyson to the Princess Andromeda.
- Percy tricks Luke into revealing that he poisoned the tree of Thalia Grace while in contact with Dionysus via Iris message. As a result, Chiron is cleared, and Tantalus is let go and sent back to the Fields of Punishment.
- Percy duels Luke aboard the Princess Andromeda.
- Percy, Annabeth, Grover, and Tyson are rescued by Chiron and the Party Ponies.
- Blackjack is freed from servitude to Luke, and Percy, Annabeth, Grover, and Tyson escape with the centaurs.
- Chiron and Percy discuss the Great Prophecy and the struggle against Kronos. Percy learns of the relationship between the Titan Lord and the centaur.
- June 19, 2010 - Percy Jackson carries Juno across the Little Tiber and enters Camp Jupiter.
- June 20, 2010 - Percy Jackson, Hazel Levesque, and Frank Zhang begin their quest to rescue Thanatos.
- June 21 - The summer solstice, when all the gods gather to have a meeting.
- June 21, 2006 - Percy Jackson duels Lord Ares for the Master Bolt and for the Helm of Darkness, winning the duel.
- June 21, 2010 - Percy Jackson, Hazel Levesque, and Frank Zhang travel to Portland, Oregon.
- June 22, 2010 - Percy, Hazel, and Frank meet Queen Hylla of the Amazons.
- June 23, 2010 - Frank, Hazel, and Percy arrive at Grandma Zhang's house in Canada.
- June 24 - The Feast of Fortuna, which determines how Camp Jupiter's luck will be the rest of the year.
- June 24, 2006 - The second Titan War starts.
- June 24, 2010 - The quest to free Thanatos is completed.
- Late June, 2008 - A Capture the Flag game turns into a search for the Bronze Dragon.
- June 25, 2010 - Demigods from Camp Half-Blood arrive at Camp Jupiter to discuss how to defeat the Gigantes and Gaea.
- Jason Grace, Percy Jackson, Piper McLean, Leo Valdez, Hazel Levesque, Frank Zhang, and Annabeth Chase are forced to flee after Leo accidentally attacks Camp Jupiter using the Argo II.
- Camp Jupiter declares war on Camp Half-Blood, causing another Civil War.
- Jason, Percy, Hazel, and Frank become fugitives of New Rome.
- Leo Valdez and Hazel Levesque meet Nemesis, who offers her help for a price, and gives Leo a fortune cookie.
- Hazel and Leo fight Narcissus and his followers for a Celestial Bronze shield.
- June 26, 2010 - Percy Jackson, Jason Grace, and Piper McLean meet Bacchus in Topeka, Kansas. The eidolons possess Percy and Jason, causing them to fight. With the help of Blackjack, Piper breaks them up.
- June 28, 2010 - The crew of the Argo II arrive in Charleston, South Carolina.
- Annabeth Chase, Piper McLean, and Hazel Levesque meet with Aphrodite in Charleston.
- Roman Demigods, led by Octavian, soon arrive and wage war against the fugitives.
- Reyna lets Annabeth go, but warns her that she would have no choice but to order an attack on Camp Half-Blood.
- The Argo II heads to the Mediterranean Sea.
- June 29, 2010 - The Argo II sails across the Atlantic Ocean.
- June 30, 2010 - The Argo II arrive at the Mare Nostrum.
- July 1, 2010 - Final day of the Quest for the Athena Parthenos.
- Annabeth Chase completes her mission for the Athena Parthenos.
- Percy Jackson, Jason Grace, and Piper McLean rescue Nico di Angelo
- Percy and Jason kill the giants Ephialtes and Otis, with the help of the god Bacchus.
- Leo Valdez, Frank Zhang, and Hazel Levesque recover the works of Archimedes.
- Percy and Annabeth fall into Tartarus after Arachne drags Annabeth to the edge of the pit and Percy tries to save her.
- Percy tells Nico to lead the others to the Doors of Death on the mortal side, located in the House of Hades.
- July 4th - Fireworks on the beach at Camp Half-Blood.
- July 5, 2010 - The quest of the Seven continues.
- Leo Valdez realizes, from clips of Camp Half-Blood on the ship, that it is the day after Independence Day.
- Leo dreams about Gaea and a shadowy figure later revealed to be Clytius.
- Leo, Jason Grace, Piper McLean, Hazel Levesque, Frank Zhang, and Nico di Angelo discuss their quest and Leo's dream - including Clytius. Nico reveals that fire is Clytius's weakness.
- The meeting is interrupted when the Argo II is invaded by the Kerkopes.
- Leo and Jason chase down the Kerkopes and apprehend them. They retrieve their stolen supplies - as well as a book stolen from Triptolemus - and Leo makes the Kerkopes promise to harass the Romans in New York.
- July 12, 1993 - Annabeth Chase is born to Frederick Chase and Athena. She is flown to her father in a golden cradle by the god Zephyros. 
- July 18, 2010 - The quest to close the Doors of Death ends.
- Frank Zhang leads a legion of Roman spirits to victory against Gaea's forces in the House of Hades.
- Jason Grace steps down as praetor, promoting Frank Zhang in his wake.
- Hazel Levesque and Leo Valdez are trapped behind a boulder during the battle.
- Hazel defeats the sorceress Pasiphaë, countering the immortal's magic with her own.
- With the help of Hecate, Hazel defeats the Giant Clytius.
- Percy Jackson and Annabeth Chase escape from Tartarus, with the help of the Titan Iapetus and the Giant Damasen.
- Reyna arrives in Greece; she, Nico di Angelo, Gleeson Hedge, and the Seven discuss the war. They form a mutual hatred of Octavian, upon learning that he defied her decree.
- Reyna begins her quest, with Nico and Hedge, to return the Athena Parthenos to the Greeks.
- August 1 - Feast of Spes, Roman goddess of hope
- August 11, 2009 - Charles Beckendorf and Percy Jackson attack the Princess Andromeda
- August 15, 2009 - Beckendorf's shroud is burned.
- Rachel Elizabeth Dare goes on vacation.
- August 16, 2009 - The Battle of Manhattan begins.
- August 17, 2009 - The second day of the Battle of Manhattan.
- August 18, 1993 - Perseus "Percy" Jackson is born to Sally Jackson and Poseidon. 
- August 18, 2009 - The Second Olympian War ends.
- Titan Army loses the Second Olympian War.
- Kronos' scythe, a.k.a. Backbiter, is destroyed by the Olympian goddess Hestia.
- Ethan Nakamura betrays the Titans and becomes the last person to be killed by Lord Kronos.
- Luke Castellan stabs himself to destroy Lord Kronos.
- Titan Lord Kronos is scattered and will most likely never be able to reform again.
- Percy and the Greek demigod forces successfully fend off the Titan Army from Manhattan and save Olympus.
- Jason and the Roman demigod forces destroy the Titan base at Mount Othrys, with Jason defeating Krios with his own hands.
- Hades and his Undead Army successfully defend the Empire State Building from the remaining Titan forces.
- Poseidon and his Aquatic Army help defeat Typhon.
- Rachel Elizabeth Dare becomes the new Oracle of Delphi after the curse is lifted by Hades.
- Percy Jackson makes the Olympians swear on the River Styx to claim their children by the age of thirteen, give Hades and the minor gods thrones on Olympus, cabins at Camp Half-Blood, and to never make a pact of the Big Three again.
- The Prophecy of Seven is revealed.
- The Great Prophecy is fulfilled.
- Percy and Annabeth start dating.
- September 2007 - Percy helps Clarisse beat Phobos and Deimos to retrieve the stolen chariot.
- September 15, 2009 - Venti continue to cause havoc after being released when Typhon was defeated.
- Aeolus orders all demigods to be killed as a result.
- September 18, 2009 - Percy and Annabeth's one-month anniversary.
- The fire-breathing giant Cacus steals Hermes' caduceus.
- Hermes seeks Percy's help to get his caduceus back.
- In a battle against Cacus, Percy and Annabeth learn that Cacus plans to cut off communication among the Olympians, as part of Gaea's plan to overthrow the gods.
- After destroying Cacus, Percy and Annabeth are sent to Paris for their date, as part of Percy's deal with Hermes.
- Around this time, Zeus has been threatening to close off Mount Olympus to mortals.
- November 2009 - Zeus recalls all the Gods back and closes Olympus, forbidding all contact to demigods and mortals.
- November 11, year unknown - Artemis and Apollo are born to Zeus and Leto.
- December 15, 2007 - Percy Jackson, Thalia Grace, and Annabeth Chase enter Westover Hall to retrieve two demigods.
- December 14, 2009 - Percy Jackson goes missing.
- December 16, 2007 - Following a traditional capture the flag match with the Hunters of Artemis, Percy and Thalia have a confrontation. They are interrupted by the Oracle of Delphi, who gives Zoe Nightshade her prophecy. At a camp counselor meeting, Zoe chooses the other four questers: Bianca di Angelo, Phoebe, Thalia Grace, and Grover Underwood.
- December 17, 1928 - Hazel Levesque is born to Marie Levesque and Pluto.
- December 17, 2007 - Early in the morning, Percy unknowingly rescues the Ophiotaurus. Zoe Nightshade, Bianca di Angelo, Thalia Grace, Grover Underwood, and Percy Jackson begin their quest to save Artemis and Annabeth Chase.
- December 17, 2009 - Jason Grace, Piper McLean, and Leo Valdez go to Camp Half-Blood and the Prophecy of Seven is set in motion.
- December 18, 2007 - After escaping Skeleton Warriors in Cloudcroft, New Mexico, Zoe, Percy, Thalia, Bianca, and Grover seek help from the Erymanthian boar. Later, Percy meets with Aphrodite and confronts Ares.
- December 18, 2009 - Jason, Leo, and Piper begin their quest to save Hera.
- December 19, 2007 - Bianca di Angelo is killed when she destroys Talos in the Junkyard of the Gods.
- December 19, 2009 - Jason, Piper, and Leo confront Medea and rescue Gleeson Hedge.
- December 20, 2007 - Zoe Nightshade reveals to Percy that she was once a Hesperid. Later, at Hoover Dam, she, Percy, Thalia, and Grover are ambushed by Skeleton Warriors. Percy meets Rachel Dare. The four questers escape, with the help of bronze angels sacred to Zeus.
- December 20, 2009 - Jason, Piper, and Leo confront Midas and Lityerses.
- December 20, 2009 - Aeolus cancels his order for all demigods to be killed.
- December 21 - Day of the winter solstice, when all the gods including Hades gather at Mount Olympus.
- December 21, 2007 - The quest to rescue Artemis is completed.
- Luke Castellan has a fall that would've killed him if he did not bear the Curse of Achilles from Mount Tamalpais.
- Percy, Grover, Thalia, and Zoë Nightshade rescue Artemis and Annabeth Chase from Dr. Thorn, the manticore.
- Zoë Nightshade is killed from both the poison of Ladon and the blow from her father Atlas, while fighting him on Mount Orthys, the Titan base.
- Atlas is trapped under the sky again.
- A new constellation is formed by Artemis from the body of Zoë Nightshade, known as "the Huntress".
- Thalia becomes the new lieutenant of the Hunters of Artemis, declaring Percy Jackson as the child of the Great Prophecy.
- December 21, 2009 - Jason Grace, Piper McLean, and Leo's due date to rescue Hera from Gaea.
- December 24, 2009 - Christmas Eve
- December 25, 2008 - Percy Jackson, Thalia Grace, and Nico di Angelo are sent by Persephone to retrieve Hades' sword from Ethan Nakamura
- The Battle of the Labyrinth is the only book in either series that are impossible to tell exactly when they take place, as no identifying dates are mentioned. However, the book takes place during the summer.