What is the Tu B’shvat Seder?

Excerpt from Hazon's Manual on running a Tu B'shvat Seder:

What is a Tu Bishvat Seder?
In the Middle Ages, Tu Bishvat was celebrated with a feast of fruits, in keeping with the Mishnaic description of the holiday as a "New Year." In the 1600s, the mystic kabbalist Rabbi Isaac Luria, of Sfat, and his disciples instituted a Tu Bishvat seder in which the fruits and trees of Israel were given symbolic meaning. The main idea was that eating ten specific fruits and drinking four cups of wine in a specific order while reciting the appropriate blessings would bring human beings, and the world, closer to spiritual perfection. The mystical kabbalistic Tu Bishvat seder has been revived, and is now celebrated by many Jews, religious and secular. Special haggadot have been written for this purpose. Hazon has put together a haggadah which is in the appendix of this manual (you can also download it from www.hazon.org).

The structure of a Tu B’Shevat Seder often corresponds to the four “worlds”–levels, realms, or spheres through which, the Kabbalists teach, the life-giving flow of the Divine is channeled and filtered. Very simply, kabbalah uses a metric which explains life through 4 worlds – the world of the physical emotional, mental and spiritual. The Tu Bishvat seder is often viewed through this lens of four worlds. All fruits are divided into categories representing the first three worlds (Asiyah-physical, Yetzirah-emotional, Briya-mental). The fourth world, Atzilut-spiritual, has no representative fruits because it is pure spirit and cannot be represented physically. Much of the seder is divided up to its corresponding kabbalistic world, often including the four cups of wine,
types of wine, and blessings.

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