Season Challenges has challenges that refresh daily, as well as ones you have the entire season to complete. Complete the challenges to gain points and unlock rewards.
There are always more Challenges than what are required to unlock all rewards. You don't have to complete all Challenges to be able to unlock all the rewards.
There are three challenges which refresh every 24 hours:
The daily challenges are always the same for each day.
The timer for refreshing the daily challenges starts when you log in to the game for the first time in the season. This is why players might have different times shown for when the Daily Challenges refresh.
Each week more challenges are unlocked.
These challenges can be completed in any order, and you have the entire season to complete them. Some challenges will take longer to complete, so you might not finish them until the very end of the season.
There are always more challenges than what are required to unlock all rewards. You don't have to complete all challenges to be able to unlock all the rewards.