Do we know yet what the plot is to Avenger's Doomsday is or what material it is lifting from? Or he is just the big bad and that's it.

well, you see, there's this guy named doom.

and well, we're told, he has a day.

All secret squirrel stuff at this point, but the sentiment is that it's leading up to Battle World:

https://marvel.fandom.com/wiki/Battleworld_(Latverion)

Doomsday is a DC character so I don't think it's about him.

But Dr. Doom may have a Day.