Hillel and his disciples declared Chamisha Asar B'Shevat a semiholiday.  They called it "Rosh Hashanah L'Ilanot" (New Year for Trees) because by this day in Israel the annual rains have ended and a new annual cycle of tree growth begins. 

Also called Tu B'Shevat (tu is an acronym consisting of the Hebrew letters tet, and vav, and has the combined numerical value of 15,) Chamisha Asar B'Shevat is celebrated on the fifteenth day of Shevat by the eating of various fruits, especially those grown in Isarel.  In modern Israel this day, also known as "Arbor (tree) Day," is celebrated by school children who go out into the fields and plant trees.  Bokser-carob has long been a popular holiday treat.