Spoilers from link

15/9/08 - Official Episode description from The CW
DEAN LEARNS THE TRUTH ABOUT HIS PARENTS IN A FLASHBACK EPISODE Dean (Jensen Ackles) is transported back in time and is shocked when he lands in Lawrence, Kansas. However, that shock turns to amazement after he runs into a young John Winchester (guest star Matthew Cohen) and Mary (guest star Amy Gumenick) who have just fallen in love. Dean isn't sure why he was sent back so he begins to enjoy the time with his parents and newfound grandfather (guest star Mitch Pileggi, "The X-Files"), who may be hiding a secret that is key to Sam (Jared Padalecki) and Dean's entire predicament. Steve Boyum directed the episode written by Jeremy Carver .


Source: SpoilerTV
3/8/08 - Without giving too much away, let's just say that Momma Winchester was keeping a LOT of secrets. Secrets that will change everything you know about, or thought you knew, about that crazy Winchester clan.


Source: Entertainment Weekly
29/7/08 - [Eric] Kripke revealed that Supernatural had just signed on X-Files alum Mitch Pileggi to play Dean and Sam's grandfather in an episode that promises a little time travel as well. Fans can also look forward to more from the Winchester past, including a story centering on when Mary and John Winchester were not only still alive, but much younger.