首楞嚴義疏注經 1.81

1.81

屬，值遇會也。聖禁三月，滿在此日，故云休夏也。自迷所犯，恣任僧舉，當悔清淨，故云自恣。制限恣法，如律所明。

"At that time" means encountering the occasion. The noble restraint of three months was completed on this day, hence it says "concluded the summer retreat." Being confused about what one has violated, one leaves it freely to the saṅgha to bring forward, so that one may repent and become pure; hence it is called "self-correction" (pravāraṇā). The regulations of the restraint and the procedure of pravāraṇā are as explained in the Vinaya.