Today is Tu B'Shevat, which basically just means "the 15th of Shevat." It's the new year for trees. Trees have a new year recognition because in Leviticus, it is said that a tree's fruit cannot be eaten for 3 years. The fourth year's fruit is a tithe to G-d. After that, you can use the fruit of the tree for whatever you want. The age of the tree is calculated on this day, so that a tree planted yesterday is already considered a year old.