Call the Guest Information Line at (714) 781-7290. You can tell the Cast Member the date that you are coming. They can give you the hours and tell you what Entertainment offerings you'll have on that day.
Good rule of thumb: If Disneyland is open until 10, 11 or 12, they are planing on being busy. If Disneyland is 10-8 then they are thinking that it will be a nice and slow day.
We were there last year on Mother's Day weekend and it was pretty busy. We'll be there again this May 12-13. I'm more concerned about how the weather will be than the crowds. I need to call Sephora and see if they are scheduling their "Mother/Daughter Makeovers" and make reservations at Catal for their brunch on Mother's Day. Everyone loved the food, but the service was terrible. So bad, they took 10% off our bill, they should have taken 20% We'll try it one more time, the food was really good.