Rhonda Hurley

Future!Dean: Okay. If you're me, then tell me something only I would know. Dean: Rhonda Hurley. We were, uh, nineteen. She made us try on her panties. They were pink. And satiny. And you know what? We kind of liked it. Future!Dean: Touché. – Dean and Future!Dean, 5.04 The End

Dean was 19 from January 24, 1998 until January 23rd 1999, which places this encounter with Rhonda Hurley before his "bendy" weekend with Lisa Braeden and well after his tryst with Amanda Heckerling, and her classmate, at Truman High.

There's no further information about what happened in that lapse of time or where should they've been except that Sam was a mathlete that year, according to Travis in 4.04 Metamorphosis.