At any point in time (except for a few minutes between seasons to allow for processing to take place), there is a current season underway.
Seasons are, on average, about two weeks long, but there have been seasons as short as 12 days and as long as 18 days.
Season length matters for the strategy. Longer seasons give the opportunity to more players to rank up as tougher opponents move to higher leagues. During shorter seasons things are more condensed and there is less time to rank up.
Recently, the seasons have been set at 15 days, without variations.
At the end of the season, players receive season rewards based on the SP they have accumulated. The number of SP they earned offers them a certain number of loot chests at the end.
The end-of-season reset rating for the next season is given by the league one finishes the season for each format, Modern and Wild, separately.
However, the type of loot chests that will be won next season is given by the highest league achieved in either of the two formats. For example, if the highest league achieved in Wild is Diamond and the highest league achieved in Modern is Gold, the next season rewards will be Diamond loot chests.