"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.
"15th of Shevat;" New Year of the Trees; Jewish Arbor Day, which is a minor festival.
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.
Coming a month and a half before the spring equinox and two months before Passover, Tu BiShvat provides a glimmer of springtime at a time when winter can often be at its cruelest.