Tu Bishvat, the New Year for trees, has implications for when to give tithes and other agricultural offerings but otherwise the day was not very widely noticed. This changed in the 16th century when the great mystic Rabbi Isaac Luria put it on the map as a minor holiday: he created the Tu Bishvat Seder, which is the day's main event.
The Seder involves eating different fruits in a way that is intended to mend the four levels of existence, which Kabbalists illustrate as a Tree of Life. The Seder includes fruits with hard shells (nuts), edible and inedible seeds, and wholly edible fruits - all symbolising our relationship with the world and our battle with evil (shells and seeds).
In the 20th century, Tu Bishvat went through further reinventions, first as a holiday marked by planting trees in Israel and celebrating our reconnection with the land, and then as an unofficial Chag "Ha-environment", a day to reaffirm our commitment to protect the natural world.