Possibly Easter because it ends up being a long, long weekend.
But as for actual holiday traditions, I would probably say Christmas. Not at all fond of the commercialisation/cheesiness/endless beating over the head with Christmas stuff for months before, but I always enjoy the actual day. I like being with my family and eating too much and giving things to people (we usually do secret santa in my family, one gift to buy and receive each, takes away a lot of the stress and expense). I like how there are virtually no cars on the roads and how you can go past houses and hear people laughing and chatting and music playing and pool splashing and kids playing with their new toys.
Christmas last year was particularly memorable, we were in the UK so while there was not a snowflake in sight, it was still my first cold Christmas, with hot roasted vegetables and Yorkshire pudding and proper flambé desserts. Totally different from the steaming hot Christmases we're used to in the Southern Hemisphere, where people often sit around outdoors in their bathing suits eating cold salads and seafood and drinking ice cold beer while the older men in the family nod off in their deck chairs in the shade.