From last night at sundown to today at sundown is the holiday of Tu B’Shevat (15th day of Shevat).
The 15th of Shevat on the Jewish calendar is the day that marks the beginning of a “new year” for trees. Commonly known as Tu Bishvat, this day marks 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.
We mark the 15th of Shevat by eating fruit, particularly from the kinds that are singled out by the Torah in its praise of the bounty of the Holy Land: grapes, figs, pomegranates, olives and dates. There is also an emphasis on nature and planting trees.
The holiday is based upon the commandment found in Leviticus 19:
23 And when ye shall come into the land, and shall have planted all manner of trees for food, then ye shall count the fruit thereof as uncircumcised: three years shall it be as uncircumcised unto you: it shall not be eaten of.
24 But in the fourth year all the fruit thereof shall be holy to praise the Lord withal.
25 And in the fifth year shall ye eat of the fruit thereof, that it may yield unto you the increase thereof: I am the Lord your God.
Tu B’Shevat was established as the way to calculate the age of trees. Obviously it would be very difficult to keep track of the date from which to calculate for each individual tree.