The mAT or Monthly Automated Tournament started a few months after the end of the Celestial Tournament, in May 2007. These tournaments no longer rely on guild position on the leaderboard to qualify, but require 20 qualifying points which you get by finishing in the top 8 of the Daily Automated Tournaments (the maximum being 8 points by finishing first to 1 point by finishing 8th).
The Monthly Automated Tournament takes place every month (last Saturday until November 2012, then the third Saturday of the month) and uses the Swiss round system, the number of rounds varies according to the number of participants. The playoffs begin immediately afterwards.