B.,
I have a 5 year old and a 2 year old but the thing that I think will make this advice most helpful is that I also have a day care in my home. I am with these kids, for most part, as much or more as their parents are so I do see them as my own children in a way. On the other hand I worked full time when my 5 year old was little so I know the feeling of getting calls like this at work.
I was picking up my little boy from day care one day when he decided to throw a fit. I saw in his face that there was something wrong so I went to him but it was too late. I had to pin him down on the ground to get him to stop. At the time I was about 7 1/2 months pregnant so I couldn't just pick him up and go to another room. But the toy that he had been playing with had been taken away by someone (one of the parents, but VERY long story) so there was a reason for his frustration.
I have had day care kids that have done the same thing. They don't understand that what they are doing is hurting the others. At this age kids don't think about cause and effect. I put them in a time out place, where I sit also to be sure they don't get up, and when the time is up we talk for a few minutes about what happened and what would have been the better way to deal with what happened.
Maybe he is upset about having to share you with his new sister. Maybe there is something going on at day care that he doesn't like.
With his age he isn't going to be able to go into detail about what he's feeling or what happened but at least he can get started. Also be sure not to discipline him for what he did hours earlier at day care. It's OVER! Just talk to him and his day care teacher about what happened and how he should deal with what happened.
I hope this helps you and remember that the day care teacher isn't always the wrong one :-)