Begins at sundown on January 24.
Tu B'Shvat, the 15th of Shvat on the Jewish calendar, is the day that marks the beginning of a “new year” for trees. This is the season in which the earliest-blooming trees in the Land of Israel emerge from their winter sleep and begin a new fruit-bearing cycle.
Tu B'Shvat, 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'Shvat, so if you planted a tree on Shvat 14, it begins its second year the next day, but if you plant a tree two days later, on Shvat 16, it does not reach its second year until the next Tu B'Shvat.