"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.
Lag BaOmer was completely off our radar when we lived in the United States. We never had any real exposure to it until we made Aliyah, and now its approach is easily recognizable by kids walking down the street, schlepping huge pieces of wood, old furniture, sticks, and anything else that burns.
Lag BaOmer is a break, a time out, a moment to recall an ancient plague that may or may not have occurred, and perhaps a moment for reflection.
The Hebrew letter equivalent of 33 is pronounced Lag (lamed gimel), giving rise to the name Lag BaOmer for this particular day. There is no one particular reason that this day stands out from the other 48 days counted between Pesach and Shavuot, yet many fascinating traditions surround the special nature of this day.