Those who follow Hillel say (and we abide by this ruling): it is on the fifteenth of Shvat."

Hillel and Shammai Make a Date
As is apparent from its name, Tu B'Shvat falls on the 15th of Shvat. ("Tu" is formed from the letters tet and vav which are equivalent to 15.) Since the holiday is not prescribed in the Scriptures, it was only in the Mishnaic period that a discussion emerged as to when to establish this New Year.

The discussion about Tu B'Shvat was led by Shammai and Hillel who headed two schools of thought in the community of Jewish scholars who lived in the first century BCE.

To fix the birthday of a tree, the rabbis picked the date that the trees stopped absorbing water from the ground, and instead drew nourishment from their sap. It was natural that Shammai would fix an earlier time than Hillel since most of his disciples lived in the coastal plain and the Sharon valley where the flowers bloom earlier than in the hills, where Hillel, and the majority of the people lived. There the soil was drier and the sap weaker. Hillel's view prevailed.

A Taxing Issue
Jewish law determined that a tenth of one's produce must be given as a tax, or tithe, which went to the priesthood or the poor, depending on the year. Originally, the rabbis viewed the New Year for Trees as the day from which the tithes (ma'aser) should be calculated, and the date from which immature fruit was prohibited (orlah). The tithe of ripened fruits had to be of the same tax year (just like today, you pay taxes from the same year's income). Consequently, fruit which blossomed prior to the fifteenth of Shvat could not be used as tithe for fruit which blossomed after that date.

The years of a tree were thus reckoned from 15th Shvat; a tree planted in December would be legally "two years old" on Tu B'Shvat, only a year and 3 months later!

A Fateful Decision
In common with the Jewish New Year in the month of Tishrei, the New Year for Trees gradually took on another dimension. The notion of trees being dependent on the amount of water in the soil naturally gave rise to the question of how much rainfall would fall in any one year. Consequently the New Year for Trees also became the Day of Judgment for the trees of Eretz Yisrael, the day when their fate is determined.

It's Time for a Holiday
Gradually, Tu B'Shvat began to take on a new look, especially after the Temple period had passed. When the Jews were exiled, they took the celebration with them. Perhaps the notion of renewal offered hope to the oppressed people; perhaps they wanted to take a little bit of Eretz Yisrael with them during their wanderings. Over the years, it became the practice to eat the fruits of the Land on Tu B'Shvat, to plant trees - especially in the modern state of Israel, and even to conduct a mystical seder service where the inner dimensions of the fruits of Eretz Yisrael are expounded, along with blessings, songs and deep discussion.