"An Unbirthday is described as an event that is typically celebrated on any of the 364 or 365 days in which it is not the person's birthday. It is a neologism coined in Lewis Carroll's Through the Looking Glass, giving rise to "The Unbirthday Song" in the 1951 Disney animated film Alice in Wonderland."
Soooo, since Gordon and Rachael are not having any sort of official "foofarah" [is that a word? Perhaps it's an unword], instead opting to travel around the world for 3 months, we shall convene in Rach's home town, with her family and just be. An "Unwedding" (neologism) it is. I'm sure there will be Champagne and Cupcakes involved:)
Caution: I can't get this song out of my head regarding Un..things. See if you are not similarly afflicted after hearing it. Just replace birthday with wedding and you've got it!