"Tu" is the pronunciation of the numeral 15 when spelled out (the letter "tet" and the letter "vov.").

The first of Nisan is called the "first month" in the Torah, commemorating a cycle of freedom for the Jews when they left Mitzrayim (Egypt). It is also called the "New Year for Kings." This was very important information in the time when Israel's monarchy ruled, because all documents were dated by the year of the king's rule. For example, if a king ascended to the throne, even a day before the first of Nisan, when Nisan rolled around it was considered his second year on the throne. Any official documents written from that day on would say, "In the second year of the reign of King So and So."

The Mitzvot relating to crops and produce are only practiced in Eretz Yisroel (the land of Israel). (Nowadays, with no Bait Hamikdash, some of these Mitzvot are observed differently than that which will be described here. However, the observance of these Mitzvot is still mandatory.)

After grains and fruit are gathered, there is a mandatory gift (tax) called "Terumah" that must be given to any person who is a Kohain, (priest). After this gift is given, there are then a series of gifts (tax) called "Ma'aser," meaning "a tenth."

There are three kinds of "Ma'aser:"

• Ma'aser Rishon
• Ma'aser Sheni
• Ma'aser Ani

After this gift has been given to the Levi, there are two other Ma'aser gifts, only one of which is taken in any particular year. (There is a system that dictates which Ma'aser is given in which year). The first of these "gifts" is called "Ma'aser Sheni" - The Second Tenth. This "gift" consists of 1/10 of the remaining crops, and it is taken by its owner to Yerushalayim (Jerusalem) to be eaten there.

The other "Ma'aser" is "Ma'aser Ani" - The Tenth of the Poor. This "gift" consists of 1/10th of the remaining crops, and it is given to poor people.

The beginning of the agricultural year for trees as far as these gifts go is the 15th of Shvat - the New Year for Trees. Any fruit ripening after Tu B'Shvat is to be assessed for tithing only during the following tax season.

For example:

The present Jewish year is 5784. If a fruit was formed on the tree before the 15th of Shvat 5784, it is included with all fruits that were formed from the 15 Shvat 5783 until 15 Shvat 5784.
If the fruit was formed after Shvat 15, 5784, it is included with the fruits formed from 15 Shvat 5784until 15 Shvat 5785 for purposes of determining to which year's gift it will be subject.
This is the significance of the New Year for Trees. Any crops harvested from the 15th of Shvat are counted for that year.

Then there's the law that states that a farmer may not harvest a tree during it's first three years (Orlah). If a farmer plants a tree on the 14th of Shvat, the tree's already a year old on the 15th of Shvat. (pretty neat trick, eh?).

In Eretz Yisroel, the 15th of Shvat is the day when new sap starts to rise in the trees. It is a time of rejuvenation. It teaches us the important lesson that even in times that seem darkest, there is new life, in times of sorrow there is hope, and in times of Galut, (exile) there is the light of Moshiach.

There are varied customs regarding eating fruit on Tu B'Shvat. Some have the custom to eat the seven species of fruits that grow in Eretz Yisroel. This "Top Seven" selection is based on a verse in Devarim: (8,8) "...a land of WHEAT and BARLEY and (GRAPE) VINES and FIG trees and POMEGRANATES, a land of OLIVE trees and (DATE) honey." Others have a custom of eating fifteen species of fruit (the "top 7" and eight more). In today’s "global fruitopia," where fruits from all over the world are available from our grocers, we mix it up: starting with the "top 7", we move on to local fruity favorites and throw in a "new fruit" (that we haven't eaten this year) in order to be able to make the Bracha (blessing) of Shehechiyanu.

For more details about Tu B'Shvat customs and their sources, click here.

Here's a story (From the Talmud - Tractate Taanit 23a) that teaches just that lesson (and something to think about while we munch on bokser fruit):

The traditional answer is found in the fascinating story of Choni Hama'agel (the circle-drawer). One day, as Choni was traveling along the road, he saw a man planting a carob tree.

Choni asked him, "How long will it take for this tree to bear fruit?"

"Seventy years," the man replied.

"Do you think that you will live another 70 years?" Choni asked.

The man replied, "I found carob trees growing when I was born, because my forefathers planted them for me, so I, too, plant them for my children."

Thereupon Choni sat down to have a meal, and sleep overcame him. As he slept, a rock formation grew around him hiding him from sight. He continued to sleep for 70 years. When he woke up he saw what looked like the same man gathering beautiful fruit from the carob tree.

"Are you the man who planted this tree?" Choni asked.

"No, I am his grandson," the man replied.

Choni then realized that he had slept for 70 years!!

Choni returned home and found that his son had died but his grandson was still alive. He said to the members of his household, "I am Choni Hama’agel!!" but they did not believe him, as seventy years had passed since Choni had last been seen.

He left and went to the Beit Midrash (study hall) and announced, "I am Choni Hama’agel," but no one believed him, and they did not show him any respect. Choni, in utter despair, prayed for Divine mercy and died. To this Rava observed, "For this reason people say, "Give me companionship or give me death!"