Tithing from trees whose fruits ripen after this date is calculated separately from those which ripen earlier. The first of Shvat is the New Year for Trees according to the school of Shamai, whereas the school of Hillel determine the fifteenth of Shvat. The festival of the fifteenth of Shvat is not mentioned in the Tanach. From this, we understand that the assignment is not the establishment of the festival but the determination of the correct agricultural calendar for fruit trees. The school of Shamai maintained that the first day of the month of Shvat was suitable, whereas the disciples of Hillel calculated that the fifteenth of Shvat was more suitable.
Custom follows the school of Hillel.
Obviously, the disagreement was not based on any inability to decide upon the festival or select an date on which to plant trees. The New Year for Trees was necessary in order to be able to implement the precept of tithing fruit. The Torah commands every Jew to take yearly tithes from the fruit of his trees and give it to the priests and Levites dedicated to the Temple services, as well as to the poor. It is forbidden to calculate the tithe from one year using produce of another year. Therefore, it became imperative to determine the date of the New Year for Trees.