The regular season often opens in March, and for the next 8 months, 12 teams battle within their own leagues to determine the top 3.
The top 3 teams from both the Central League and the Pacific League then play against each other to claim the number one spot in their leagues. This series of matches is called the Climax Series, and begins in late October. The number one team from each league then face-off at the Nippon Series, a best-of-seven series held in November, which determines the champion of the season.
Other dates worth noting are the Interleague Games, where for a limited time, teams from one league play teams from the other. 2020’s match-ups are from May 26 to June 14, with the All-Star Games taking place on July 19 and 20.
You can see the complete schedule for NPB games on their English website here.