New Year's, Easter, (U.S.) Memorial Day & Independence Day, Halloween, Thanksgiving, & Christmas. Also sometimes Cinco de Mayo & Mexican Independence Day, & Chinese/Lunar New Year & Moon Festival with friends. If someone I know invites me to a good PG-rated Solstice Celebration I'll do that too on occasion.
Major ones for my family & friends are Thanksgiving, Christmas, & New Year. Thanksgiving is lots of food, football, movies, & games - all weekend long. Christmas is presents for the kids, food (Mmm, home-raised beef & corn!), and together time. My mother grew up in Pennsylvania, so New Year's lunch is Pennsylvania Dutch - pork & cabbage! - and all the bowl games we can watch.