CN rushed the final Regular Show season so hard that they haven't even finished the final work on the finale yet.

https://twitter.com/SeanSzeles/status/817199233090490369

So basically it's airing as soon as it's completed next Monday. That's the reason it's airing on a Monday during a Teen Titans all day marathon, instead of at a better time like on the Network Premiere day Thursday, which has no new episodes that week so it would've been a perfect placement.