Tu B’Shevat begins at sunset, tonight, and ends at nightfall tomorrow, February 4th.” It is The New Year for the Trees.
Tu B’Shevat is the new year for the purpose of calculating the age of trees for tithing. See Lev. 19:23-25, which states that fruit from trees may not be eaten during the first three years; the fourth year’s fruit is for G-d, and after that, you can eat the fruit. Each tree is considered to have aged one year as of Tu B’Shevat, so if you planted a tree on Shevat 14, it begins its second year the next day, but if you plant a tree two days later, on Shevat 16, it does not reach its second year until the next Tu B’Shevat.