I wouldn't announce it ahead of time to him. But, as you are going there, just tell it like it is. "You are going to daycare while I go to work. You will eat lunch, play, and take a nap. Then I will come back to get you."
Can he take a favorite toy with him?
Try to phase it in, if possible. Just leave him for an hour at first, if possible.
All toddlers will be upset when you leave them. That is normal. What you need to pay attention to, though, is how he looks/reacts when you pick him up. He should be happy that you are back, but also happy that he's had some time to play. He shouldn't look totally stressed out when you pick him up. (If he continually looks stressed out when you pick him up, then you may need to find other care. )
Good luck! It's hard on mamas, I know!! You can also call the daycare while you are at work to check on him.