I am familiar with a Passover seder but what is a Tu BiSh’vat seder?
Tu BiSh'vat (Hebrew for the 15th day of the Hebrew month of Sh'vat) is the new year of the trees. This year, Tu BiSh'vat begins at sundown on Tuesday, January 30th.
The sixteenth- and seventeenth-century kabbalists (mystics) created this ritual, modeled after the Passover seder. They would gather for a multi-course meal with fruits that are associated with the Land of Israel. Commonly included foods include fruit from three different classifications: fruit with shells (oranges), fruits with pits (dates), and fruit that are entirely edible (raisins).