Seen a bunch of peeps makin' these...how about I give it a try?
January: You and X open the mysterious box while Y watches.
February: You help Y fight X.
March: You see X in concert with Y.
April: You go through the magical mirror with Y and meet a human version of X.
May: X and Y argue over which movie to watch for movie night while you try to settle them down.
June: You help Y make a dress while X is the model.
July: Y turns out to be a wizard pony and turns X into a batpony.
August: You help and X and Y out in Sweet Apple Acres.
September: You help Y fall to sleep while X watches.
October: You, X, and Y celebrate Nightmare Night.
November: Y leaves you and X to petsit their pet.
December: You celebrate Hearth's Warming Eve with X and Y.
X (Day Of Which You Were Born)
1: Braeburn 2: Cheese Sandwich 3: Babs Seed 4: Rarity 5: Seabreeze 6: Snips 7: Scootaloo 8: Cheerilie 9: Zecora 10: Gummy 11: Daring Do 12: Fluttershy 13: Rainbow Dash 14: A.K Yearling 15: Pinkie Pie 16: Derpy Hooves 17: Lyra Heartstrings 18: Bon Bon 19: Discord 20: Apple Bloom 21: Sweetie Belle 22: Applejack 23: Dinky Hooves 24: Berry Punch 25: Doctor Whooves 26: Twilight Sparkle 27: Sunset Shimmer 28: Flash Sentry 29: Thunderlane 30: Photo Finish 31: Sapphire Shores
Y (Year Of Which You Were Born)
1970-79: Spike 1980-89: Trixie 1990-99: Queen Chrysalis 2000-09: Big Mac
Fluttershy and Big Mac argue over which movie to watch for movie night while you try to settle them down.
Hmm. Kinda makes sense...
Sorry for the characters who couldn't make it into the birthday game. Post away in the comments!
EDIT: I changed the roles of X and Y in the April situation.
EDIT 2: I also changed the roles of X and Y in the March situation.