End of Season? Contract Negotiations?

The regular season ends in early to mid October. There are then the playoffs and Nippon Series held from mid October to the end of October. The end of the Nippon Series is the critical date for declaring free agency. (Not part of your question, but a critical date for Japanese player contract negotiations.) The Asian Series follows the Nippon Series and is held at the beginning of November to decide the best team between Japan, Korea, Taiwan, and an all-star team from China. But this series is beyond post-season.

Most foreign players are told several weeks before the end of the season if they're going to be brought back for another season or not. And from my understanding, some also have clauses in their contracts that require the club to have an agreement for the next season before a given date, often the end of September - well before the end of the season. Failure to come to an agreement before that date will result in the player essentially being released and free to go wherever he (or his agent) wishes.