Harvest Festival, even though it sounds like it takes a lot of time. I've done Festivale and April Fool's in my TT town, and they're really fun! Festivale takes a long time by yourself unless you're really lucky, so I'm sure the boards will be blowing up with people wanting to make trades.
Defintely Christmas, my favorite holiday in real life. Plus the trees look so awesome.
Isn't it called Toy Day though? I dunno...sounds kinda dumb, likely due to the use of the word "Christ" which alienates non-christian players I guess? Wish Nintendo wasn't so touchy.
Mine is currently Halloween. This time of year was always my favorite when I was a kid. But I easily forget all about the excitement of Halloween after it's passed, so after that, I'll be looking forward to Christmas! I also can't wait for Easter, but Christmas sounds cooler to me than Easter.