Tu B'Shvat is the Jewish "New Year" of the trees. On Tu B'Shvat we plant trees For future generations. Tu is the number 15 in the biblical numbering system (which uses the letters of the Hebrew Alphabet). B' is Hebrew for "in". Shvat is the current month in the Hebrew calendar. So Tu B'Shvat simply means the "15th of Shvat".
This year Tu B'Shvat starts on the evening of Sunday January 24 2016, and ends on Monday evening January 25,