"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.
The sukkah is a symbol of fragility. We build the temporary structure each year and know that it is only meant to last for the week-long holiday. It sways in the breeze. The raindrops land inside. The animals nibble at our decor. We know it could come crashing down on us.
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.