This is a canon-divergent extension of the 4th chapter of More Than a Day More Than A Word. Reading that chapter before this is suggested, but here's a quick refresher:

it's Valentine's Day 2005, Pam is not happy about an insensitive comment Roy made regarding their continued wedding delays, but is cheered up when Jim gives Pam a goofy card (mentioned the the S2 V-Day episode). She responds with a card of her own and asks Jim what his Valentine's Day plans are. He tells her his friend is hosting a "singles only" party and Pam inquires about going as she's still irritated with Roy and not interested in spending v-day with him. But when Roy figures out Pam's mad at him and apologizes, her plans to attend Jim's friend's party are scrapped (though she is a bit disappointed and senses Jim's disappointment as well)

oh and disclaimer: Jim, Pam and the Office do not belong to me