Wet season is when the monsoon comes down from the equatorial areas and moves down south - and correspondingly it pisses with rain !!!

Other factors involved make the wet season the wet season
but you get the drift.

It varies quite a lot from year to year, this year it has been an extremely dry start - very little rain up until Xmas day. The year before it pissed down all through December etc - at least where I was.
They also had a cyclone that didn't help !!!