"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.
It is life we want, no more and no less than that, our own life feeding on our own vital sources, in the fields and under the skies of our homeland, a life based on our own physical and mental labors; we want vital energy and spiritual richness from this living source.
It is a truth universally acknowledged that it can be difficult to be Jewish at Christmas time. It has seeped into North American cultural consciousness so thoroughly that South Park even wrote a song about it, complete with trademark expletives.
"15th of Shevat;" New Year of the Trees; Jewish Arbor Day, which is a minor festival.