The first day of every spring when the winter weather parts and the first hint of warmth brings people outdoors in droves. The people, whom you've curiously never seen before had effectively been in hibernation and their emergence triples the population of your town, city, college, etc. Sun on That-Day is key. Activities often include frisbee, hacky-sac and basking in the sun. The effects of That-Day are most evident on college campuses.
Hey, I think it's That-Day today! Cool, let's skip class and play frisbee!