The timeline of Supernatural is quite complex, covering numerous key events and character developments. Here is a detailed timeline based on official sources and fan consolidations:
Mythological Arc Timeline
Beginning of Time:
God and his sister, Darkness, are born.
God, against Darkness's wishes, splits apart, causing the Big Bang and forming the universe.
God and Darkness wage war, with God eventually defeating and imprisoning Darkness with the help of archangels, creating the Mark as both a lock and a key to ensure Darkness remains imprisoned.
The Four Horsemen are born.
God creates the Leviathans but, fearing they would destroy his other creations, imprisons them in Purgatory.
God creates angels, who will reside in Heaven, forming the angelic society known as the Heavenly Host.
God creates Hellhounds as companions but, finding them too vicious, destroys them all except for one pregnant female, Ramsey, whom Lucifer saves.
God infuses himself into the framework of the universe so that he can leave to create other universes.
Castiel joins Anna's garrison and is stationed on Earth. He witnesses the first fish crawling onto land and is told by an older angel not to step on it because God has big plans for this fish.
Grigori are among the first angels stationed on Earth to observe and protect humanity. However, when they begin consuming human souls, they are believed to have been destroyed.
The Mark begins to have a will of its own, starting to corrupt Lucifer, amplifying his jealousy of humanity. When God instructs the angels to bow down to humans, Lucifer refuses, considering them flawed and unworthy. Lucifer asks his brother Michael to support him, but Michael refuses.
Gabriel, tired of the family squabbles, leaves Heaven and hides on Earth. He finds Loki imprisoned in a cave and releases him, striking a deal where Gabriel pretends to be a trickster.
Lucifer convinces Gadreel to let him into Eden/Earth.
Lucifer rebels against God, depriving humans of their humanity and creating demons. The first demon is Lilith.
God instructs Michael to cast Lucifer out of Heaven.
The first generation of demons, born after Lilith, are known as the Knights of Hell, intended to lead the armies of Hell against Heaven.
Lucifer begins communicating with Abel, and when Cain realizes Lucifer's plans, he makes a deal with the archangels: Abel's soul in Heaven, and his in Hell. Lucifer accepts the terms, on the condition that Cain must kill Abel.
Lucifer passes the Mark to Cain. Cain, unwilling to be the killer the Mark wants him to be, attempts suicide with the First Blade but is resurrected as a demon.
Lucifer selects humans for Cain to turn into demons (including Abaddon) and has him create the order of demons known as the Knights of Hell.
God, seeing all of this, imprisons Lucifer in the deepest pit of Hell, sealing him with six hundred seals, sixty-six of which must be broken for Lucifer to escape.
Season 1 Timeline
October 30 - November 2, 2005: 1.01 Pilot
Days: 4 days
Locations: Palo Alto, California; Jericho, California
Dean arrives in Palo Alto late at night. The brothers spend 2 days investigating the White Lady in Jericho. Jessica dies. Sam and Dean hit the road.
October 30 (Friday afternoon) – We are introduced to 22-year-old Sam, who is attending Stanford University in Palo Alto, California, and his girlfriend is Jessica Lee Moore. They attend a local bar Halloween party, and we learn that Sam is preparing for a law school interview on November 2 (Monday). (Note: October 28, 2005, was actually a Friday; the following Monday was actually October 31. But clearly Sam returns on Sunday night (before his law school interview on Monday), and Jess's tombstone (shown in subsequent episodes) indicates her death date as November 2. So either the night of November 2 or the morning of November 2. Anyway, it's an unreal calendar.)
October 31 (Saturday morning) – We are introduced to Dean, Sam's older brother, who breaks into Sam and Jessica's apartment. He convinces Sam to join him in searching for their missing father, John. Sam leaves, promising Jessica that he will be back in time for his law school interview on Monday.
October 31 (Saturday) – Sam and Dean arrive in Jericho, California, searching for their father and tracking his work (the White Lady) there. Dean ends up in the river by the end of the day.
November 1 (Sunday morning) – Dean is arrested by local police, who consider him one of at least two serial killers suspected in the Jericho disappearances. Sam goes to the widower of Constance, Welch, and they identify her as the "White Lady."
November 1 (Sunday afternoon) – Dean escapes from jail and calls Sam to tell him their dad left unknown coordinates in his journal. They later meet up when Sam is threatened by Constance. The final confrontation takes place in her former home, where she is dragged to Hell by the ghosts of the children she murdered.
November 1 (around 10:00 PM) – They leave Jericho. Sam identifies the coordinates as Blackwater Ridge, Colorado, but insists that Dean drop him back at Stanford.
November 2 (Monday, 2:50-3:00 AM) – Dean drops Sam back at his apartment, where he witnesses Jessica dying in the same way as his mother did twenty years ago on the same day. Dean returns in time to drag Sam out of the burning apartment. (Time from deleted scenes on DVD/BluRay)
Mid-May 2006: 1.15 The Benders
Days: 3 days
Locations: Hibbing, Minnesota
Sam is kidnapped by The Benders in Hibbing, Minnesota. The brothers spend 3 days there (Sam is missing for 2 days).
Late May 2006: 1.16 Shadow
Days: At least 3 days
Locations: Chicago, Illinois
Sam and Dean discover someone summoning a Shadow Demon in Chicago, Illinois, and meet their dad there.
July 15-16 or August 1-2, 2006: 1.22 Devil's Trap
Days: 24 hours
Locations: Savannah, Iowa; Sioux Falls, South Dakota; Jefferson City, Missouri
Sam and Dean travel from Iowa to South Dakota, using a Devil's Trap to find their dad's location, then they drive to Jefferson City, Missouri, to launch a rescue operation.
Recent Past
Fall 2009: 5.09 The Real Ghostbusters
Becky tells Sam that Bela lied about giving the Colt to Lilith; she actually gave it to a demon named Crowley.
January 2010: 5.10 Abandon All Hope...
The Colt is reclaimed from Crowley.
Sam, Dean, Castiel, Ellen, and Jo hunt Lucifer.
Ellen and Jo are killed.
The Colt fails to kill Lucifer because he is one of the five creatures in creation that the gun cannot kill.
Lucifer resurrects the Four Horsemen.
January/February 2010: 5.13 The Song Remains the Same
Sam, Dean, and Castiel follow Anna back to 1978, where she attempts to kill John and Mary Winchester.
February 14, 2010: 5.14 My Bloody Valentine
Famine causes people to satisfy their desires in often fatal ways.
Sam drinks demon blood again, expelling all of Famine's demons.
They take Famine's ring.
March 2010: 5.16 Dark Side of the Moon
Sam and Dean are killed by hunters and go to...