Details for rule "Seas-1" (Season winner)
The duration of a running season is stated in the description of a division.

A season is won by the team with the most played matches on rank 1 when the season is closed.
In other words, assuming that all teams play an equal amount of matches a week, this is the team that was the longest on rank 1.
At the end of a season stats are gathered and the winner is announced on the awards page.