OriginsIt is mentioned in the Bible, sort of, but not specifically as a festival. More on that in a link tomorrow.
Tu Bishvat, the New Year for Trees, is a minor festival and one of four new years – the others being Rosh Hashanah on Tishri 1, the start of the calendar year on Nisan 1 and the least known, Ellul 1, the new year for tithing cattle in classical days (akin to the start of the tax year).
Unlike the other new years, Tu Bishvat occurs mid-month rather than at the beginning; its name simply refers to the date on which it falls, the 15th day of the eleventh month, Shevat. Although not mentioned in the Bible, it is referred to as Rosh Hashanah L’Ilanot, New Year for Trees, in the Talmud and marked the date on which farmers began calculating the tithing of their fruit crop for the coming year.
UPDATE: Yes, I did mean tomorrow, which is when the festival starts. The post appeared prematurely today, but that was due to a pre-posting that went awry. Apologies for the error.