Tu B'Shevat, the 15th day of the Jewish month of Shevat, is a holiday also known as the New Year for Trees. The word "Tu" is not really a word; it is the number 15 in Hebrew, as if you were to call the Fourth of July "Iv July" (IV being 4 in Roman numerals). See Hebrew Alphabet for more information about using letters as numbers and why the number 15 is written this way.
As mentioned in Rosh Hashanah, the Torah has several different "new years". This is not as strange a concept as it sounds at first blush; in the US, for example, there is the calendar year (January to December), the school year (September to June), and many businesses and governmental institutions have fiscal years. It is basically the same idea with the various Jewish new years.
Tu B'Shevat is the new year for the purpose of calculating the age of trees for tithing. See Leviticus 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 God, and after that, you can eat the fruit. A tree is considered to have aged one year as of Tu B'Shevat, only if it was already planted 44 days before Rosh Hashanah (the normal Jewish New Year on the first of Tishri); then it begins its second year on Tu B'Shevat, about six months after planting. But if it was planted just a day later, it begins its second year about a year and a half later, on the second Tu B'Shevat after planting.
There are few customs or observances related to this holiday. One custom is to eat fruit on this day, especially fruit from the Land of Israel. Some people plant trees on this day. A lot of Jewish children go around collecting money for trees for Israel at this time of year. That is about all there is to it.
Tu B'Shevat occurs on the following days on the civil calendar:
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