Synonym for "birthday". It originates from the word pizda(which means "cunt"/"pussy"/"slit" in romanian, russian and maybe other languages too) + "day", because your birthday is after all the day you came out of your mother's pizda.
Jimmy: hey Bimmy, sup?
Bimmy: yo it's my pizday dude!
Jimmy: holy shit! Happy pizday, douchebag! Buy me beers.