Should be Dec 1. Its usually been on the 1st of the month after GST (which is last saturday of the month); So Dec 1, March 1, June 1, Sept 1.

// but anyway Im glad the amount of riot boosting has been pretty low this last season. Ive only been online a few days a week, but from what I can tell, it doesnt look like there are a ton of people who have a ridiculously inflated number of kills for one season.