Saturday is Tu B'Shevat, known as the "Jewish New Year for trees," the "Jewish Arbor Day," or the "Jewish birthday for trees." The holiday has an interesting history that, believe it or not, began with taxes. Lenore Skenazy explains in The Forward:
Back about 2,000 years ago, Tu B’Shevat — literally the 15th day of the month of Shvat — was a tax deadline, of sorts. Any trees planted before Tu B’Shvat were considered to have been “born” the previous year. Those planted after Tu B’Shvat (or, perhaps those that started blooming after Tu B’Shvat) were part of the next year’s crop. As the amount of fruit you were required to tithe from each tree was determined by its age, this date was significant. And since the easiest way to remember a tree’s birthday was to plant it on that day, that’s what some folks did: planted.