Tu Bishvat is the New Year of the Trees, a holiday with roots (pardon the pun) dating as far back as the Talmud. But, unlike Passover or Purim or Sukkot, there’s no prescribed way to observe Tu Bishvat.
Not unless you ask the Pri Etz Hadar, that is. The Pri Etz, a mystical text dating back to the 1600s, was popular in Sephardic circles–for centuries, it was virtually unknown to most Ashkenazic Jews. In modern times, together with other Sephardic works, the Pri Etz has been embraced by Ashkenazim.
Much of the Pri Etz‘s content is philosophically complex (it deals with the appearance of evil in God’s world, as symbolized by the fusion of inedible seeds and rinds with edible fruit). Other parts, discussing human sexual metaphors, might be considered risqué. Its most well-known contribution is a Tu Bishvat seder, in which participants drink four cups of wine (or grape juice) and eat fruits, working their way through different kinds to explore the panoply of foods that trees provide us with.
The text of the Pri Etz dates back to the times of Shakespeare, but its lessons–and its fruit–are as digestible today as they ever were.