The Title Stage Playoff Matches are the only matches in the Overwatch League which are held on a Sunday. After 5 weeks of grueling league matches, the top three teams compete in the playoffs for a chance to win a cash bonus and the title of being the Stage champions.
This is what the Championship Sundays are according to Nate Nanzer.
https://twitter.com/natenanzer/status/978063897922908160?s=21