Growing up with Norwegian heritage, I've always had a special place in my heart for May 17th... Norwegian Independence Day, which mainly seems to involve sitting around cursing Swedes.
When I was in Korea, Buddha's birthday was a big deal - April 30 or thereabouts, as was the Korean version of "Thanksgiving" (their choice for translation, not mine). More aptly described as an Ancestral Remembrance Day, the point is to go to the graves of relatives, bow ceremoniously, and leave offerings of food and flowers. With a roughly 50/50 split among Buddhists and Christians, this seemed to be the most unifying national celebration they had. I was partial to Buddha's birthday, though. Lots of lantern-lighting and biben-bop (a vegetarian rice dish traditionally served for the holiday).