Baseball Seasons?

The main professional season starts at the end of March and goes through the middle of October. This is after "Spring" training that starts in February. The Pacific League starts their playoffs at the beginning of October, with the Nippon Series behing held at the end of October.

Starting in 2005, there is also an Asian Championship Series between the professional leagues in Japan, Korea, Taiwan, and Mainland China. This is held at the beginning of November.

The MLB also sends over some All-Stars for a tour of Japan every other year at the beginning of November.

Most amateur leagues (high school and college included) play all year round with major tournaments in the Spring, Summer, and Fall. The two high school tournaments at Koshien are perhaps the most famous, with the Senbatsu in April and Natsu Taikai in August.

There are a number of different types of baseball played by kids below high school. There's the standard Little League (with ties to the U.S. organization), as well as nanshiki (rubber ball) baseball and softball. My second grade son is playing in one of the nanshiki leagues, and they practice every Saturday, Sunday, and national holiday all year round (except for a couple of weeks for the New Year and summer vacations). His group only practices in the mornings right now, but half way through fourth grade they start practicing all day - morning and afternoon. They know nothing about weekends outside of baseball.