The Babylonian Talmud distinguished four New Years and one of them is Tu BiShvat (the New Year of the Trees), which in ancient Israel was when the counting of agricultural crops began. The name of the holiday is derived its date- the 15th of Shevat which in Hebrew is written down using the letters tet ט and vav ו, with the geometrical value of 15[ט"ו = ט(9)+ו(6)=15]. The letters yod י and heה, whose sum is also 15, are purposely avoided because combined they form one of the names of God.
Work is not prohibited on the holiday, and there are no liturgical changes. The eve of the holiday is the most important part- after sunset, a traditional but simple meal (also called a seder, meaning “order” or “sequence”) is eaten, the main element of which is consuming the Fruits of the Land of Israel (however, in the diaspora these need not be fruits imported from Israel). This seder has particular importance to Kabbalists who imbued it with mystic significance connected to the idea of “the Four Worlds”. Each world symbolises different levels of spirituality and piety.
The first and lowest realm is olam ha-‘asija (עולם העשיה), that is the “World of Action”. During the seder, it is symbolised by nuts and oranges, as well as any fruit with an inedible exterior.
The second, slightly higher, level of spirituality is olam yetzirah עולם יצירה – „World of Formation”. It is represented by any fruit with seeds or pits- grapes, plums or apples.
The third and fourth levels are olam briah עולם בריאה, “the World of Creation” and olam atzilut עולם אצילות – „the World of Emanation”. These are purely idealistic realms represented by fruit without a rind or seeds- that is, fruit which does not de facto exist, and is beyond the earthly realm.
Four cups of wine are also drank during the seder; like the fruit, they also represent the four levels of spirituality, but also the yearly cycle of nature. The first cup should contain only white wine, symbolising winter. The second cup should contain white wine with a small amount of red wine- representing spring, when plants slowly begin to come to life. The third cup is filled with red wine and a small amount of red, representing the hight of spring. The fourth and final cup contains only red wine, which symbolises summer.
In modern-day Israel, Tu BiShvat is a time of planting trees, and the holiday itself has taken on a distinctly zionist character and is the symbol of returning to Eretz Yisrael and its abundance of agricultural crops.