John's Journal is one of the iconic items of Supernatural. It provides an invaluable source of information about supernatural creatures and lore, and is a symbol of the Winchester "family business."
Journal keeping appears to be a hunter's tradition -- other hunter's journals include those kept by Samuel Colt, Moishe Campbell, Daniel Elkins, Bobby Singer, Gordon Walker and Jo Harvelle. The Journal was originally purchased by Henry Winchester, John's father, to use in documenting the knowledge he gained as one of the Men of Letters. It was delivered after he had disappeared (having traveled to 2013 and died unbeknownst to John). Inside the front cover the initials H.W. are embossed.
John kept this Journal from early on in his hunting life. At the age of eight, it is from John's Journal that Sam finally discovers the truth about supernatural beings, and what his father's real job is.
The Journal comes into Dean and Sam's possession when John leaves it in his motel room in Jericho, California before disappearing. The Journal (which Sam and Dean most frequently refer to as "Dad's Journal") seems to hold a large body of John's hunting knowledge, though not all. As time goes on, Sam & Dean comment on the information that John has left out of the Journal. Nevertheless, Sam and Dean frequently use it as their first point of reference when investigating a job. Interestingly, Sam and Dean never start their own journals.
The Official Website had a reproduction of John's Journal.
After the wendigo has taken all of their equipment, Sam takes Dean aside and asks for John's Journal that he opens to a page that shows John's entry on wendigos that has a drawing that John made along with all of the Anasazi symbols that the wendigo cannot cross.
The boys find a reference in John’s Journal to kids who disappeared from the south wing of the Asylum in 1972.
While investigating disappearances in Hibbing, Minnesota, Sam reads that in the Journal, John suggests the area could be the hunting grounds for a phantom attacker which is what he believes is causing the disappearances.
After Dean explains that what they are dealing with is a shtriga that John had failed to kill years before, Sam discovers that he wrote nothing about it in the Journal.
John's Journal records the history of deaths related to the painting. Sam connects up John's pattern with the painting's Provenance. This page we glimpse of the Journal appears in Jo's Blog, captioned "page from Gordon's logbook."
After learning of the death of Daniel Elkins in Colorado, Dean recognizes the name and finds Elkins address and phone number in John's Journal, causing them to realize that Elkins may be a hunter and friend of their father and they decide to investigate.
Sam speculates that what's happening to people could be related to a theory in John's Journal, that Croatoan is the name of a demon associated with plague and pestilence, and calls what's going on "demonic germ warfare."
Sam finds lore in John's Journal that suggests that if the “sire” of someone who is a werewolf is killed, they will be released from the curse. Sam reads from the Journal: “Lycanthropy might have a cure if you kill the werewolf who bit you severing the bloodline” (It is later revealed that Bobby already knew that this theory wouldn't work, but it is unclear if John was ignorant of this or just never updated his journal to reflect it). Sam and Dean's comments in the episode suggest they already knew of the theory and just consulted the Journal to double-check exactly what it said.
Sam reads from the Journal to summon the captain of the Espírito Santo.
In a flashback to 1991, Sam (aged 8) finds John's Journal, and discovers that monsters are real and the family's role in hunting.
After realizing that Doc Benton is the culprit, Sam shows Dean the entry in John's Journal on John's hunt for the doctor.
Dean has the Journal with him when Castiel sends him back to 1973 and he uses it to track where Azazel will attack next. He shows Samuel how John documented all possible signs related to and appearances of Azazel, but claims that his father could see the future to account for some future dates.
Dean reads the Journal while he and Sam prepare a ritual in a graveyard to speak to the last person to die in the town of Greybull, Wyoming.
Sam uses the Journal to point out to Dean that Adam Milligan might, in fact, be their half-brother. A page that's dated close to the time that Adam would have been conceived has been torn out.
While researching lore on dragons, Sam is going through John's Journal looking to see if John made any entries on dragons to which Dean says there isn't, and Sam has a deja vu moment when he's sees John's entries on skinwalkers and asks Dean if they hunted one lately to which Dean says no since he doesn't want to prompt any of Sam's memories about what happened when he didn't have his soul.
Sam uses John's journal to find out about vetalas. While there is information on how to kill them, John encountered a single creature, whereas they usually hunt in pairs. This means Sam is unprepared when he tracks one of them down.
While Sam is on their laptop researching, Dean is studying John's Journal looking for any monster or spirit that leaves a mummified corpse behind to which he replies, "Nothing that turns a dude into a Cryptkeeper."
While trying to figure out what could be causing the weird deaths in Wichita, Kansas; Dean can be seen looking in John's Journal for clues to no avail.
Looking for what monster has claws and is invisible as one is attacking kids in Junction City, Kansas; Dean looks in the Journal for information, but isn't able to find anything.
Dean looks through the Journal while trying to find what is causing people's hearts to be ripped out. Later, he finds information on the Mayans and Cacao in the Journal, leading them to realize that Brick Holmes made a deal with Cacao which is what led to the situation they are investigating.
While investigating a case Sam and Dean research in John's Journal and discover that "werewolves that are turned up to four generations from pureblood are less feral and can transform before, during, and after the lunar cycle." They also discover that John noted that pureblood werewolves can control themselves and can survive on animal hearts.
Looking for information on the demon Abaddon, Sam searches through John's Journal and discovers that John tortured a demon that worked for Abaddon and learned that Abaddon is a Knight of Hell. While examining the Journal, Henry Winchester discovers that it is the one he ordered to be his own the day before he disappeared. Henry reads through the Journal and feels guilty for abandoning John and decides to go back in time to prevent this from ever happening. When he tells Dean that he can't understand how he feels after reading it, Dean says he can because he has read the Journal more times than Henry can imagine and he feels pain every time he reads it.
Looking for information on what could turn a man's insides to jelly and leave a blue handprint, Dean checks the Journal while Sam and Charlie Bradbury check other sources. Eventually Dean finds out from the Journal that they are dealing with a "bastard off-shoot" of a djinn which can be killed the same way. Charlie expresses envy about the Journal saying she hates it and wants one.
Searching for information on a hunt John conducted where he interrogated a demon working for Abaddon who knew about the First Blade, the only weapon that could kill a Knight of Hell, Crowley approaches Dean for help and he checks the Journal. In the Journal, John mentions he tortured the demon, but doesn't leave any specifics besides it worked for Abaddon and he exorcised it. He also writes in the margin a group of numbers which refers to his lock-up and a T which refers to Tara, the hunter he worked the case with.
Sam brings a resurrected Mary Winchester the Journal, suggesting she read it as it could fill in some of the blanks for her. Sam admits that reading John's Journal filled in some blanks for him that he didn't even know he had. After Sam leaves, Mary opens the Journal and begins to cry looking at the contents.
Late at night Mary is seen by Castiel reading John's Journal in the Bunker library. Later, when Mary tells Sam and Dean she needs to leave them for some time, she walks out with John's Journal in hand.
After meeting up with her sons again at Asa Fox's wake in Canada, Mary tells them that after returning to Lawrence briefly, she retraed a few of the things in the journal in her efforts to understand the world she has been resurrected into.
After discovering that Jack Kline is in Jasper, Wyoming, Sam looks through the journal and discovers that Jasper has a Hell Gate that leads to "a place where unimaginable evil emanates from creatures too wicked for the Pit to hold."
When Mia Vallens asks if the Winchesters keep a journal, Sam mentions that their dad did while Dean only gives a sarcastic response.
Sam looks through the journal for information on dreamwalkers. When Dean joins him, Sam informs Dean that the lore on dreamwalkers is light.
John's Journal can be seen on the map table at Mary's funeral service.
After Jack suggests a spell to open a portal to the Bad Place, the Winchesters reveal that John stated in his journal that he had hunted and killed the last of a species of monster -- the mandragora -- they need for the spell.
The journal is seen among the various books Sam is going through while researching.
After learning that vampires kidnapped a couple's children and were wearing masks, Dean recognizes the wife's drawing of the mask from John's journal. Looking through the journal on the hood of the Impala, Dean finds the entry on the case in 1986 when John was working a series of kidnapping along Route 77 in Akron, Canton and East Sparta, Ohio. Although John hadn't found much, the witness drew the vampires' masks which is also included in the journal entry. The journal states that the kids were taken and the adults that weren't killed had their tongues ripped out.
Sam deduces that if it is the same nest and the pattern holds, the vampires will target Canton next. Dean reads from the journal that the nest targets families living outside of town that are isolated and with kids usually between the ages of five and ten. By determining who in Canton matches the MO John noted in his journal, Sam and Dean are able to ambush the vampires at their next target and force one to lead them to the nest where they take out the remaining vampires and rescue the kids.
The Journal in Tie-ins
The Official Website was home to an electronic version of John's Journal. The first handful of pages have diary entries written by John immediately following Mary's death, the website implies John started the journal the day of Mary's funeral. You can read a transcription here.
Following the "diary entry" pages are mocked-up pages of the journal containing notes, newspaper clippings and other scribbles. You can find transcriptions & screencaps here.
This book by Alex Irvine is a reproduction of John's Journal from 1983 until the events preceding the "Pilot." It includes extensive notes on John's hunts, and tracks the family's life over these years.
The Supernatural tie-in comic, Origins, contradicts the Official Website and shows canon; John is given the blank journal by a character called Fletcher Gable, who he visits after he meets Missouri Moseley.