The Dutch football association KNVB released the program for the 2020-2021 Eredivisie season. You can find it here

Please contact Willem II directly if you are looking to buy tickets to watch a match at the Koning Willem II stadium. 

The A, B and C stands behind the goal at the north side of the stadium (King Side) are completely sold out throughout the season, but you might be able to obtain tickets for other stands. However, D is reserved for fans of the opponent.