"Tu" (the Hebrew abbreviation of 15th) in the month of Shvat was set (Hillel's opinion generally overrules Shammai's) as the beginning of the tithable year for tree fruit: Calculating the tithe on fruit starts again for fruit that sets after that date.
Here is the quintessential Jewish question: How do we emulate God? We are told that we were created by God. We are told that we have a divine spark within us.
Jewish communities around the world marked the "new year for the trees" last week with tree planting ceremonies and seders that celebrate Israel's seven species (wheat, barley, grapes, figs, pomegranates, olives and dates if you are keeping track!).
"15th of Shevat;" New Year of the Trees; Jewish Arbor Day, which is a minor festival.