thats not an ER. ER's have to be staffed 24/7 by an MD or maybe in a tiny rural one a PA or Nurse Practioner. I will bet you it was an Urgent Care, which can operate any time they please.
Depending on the type of insurance you have, an HMO for instance, an “Urgent Care” facility may not be covered. If your doctors office is closed and the hospital ER won't see you and all you have left is an independently operated, non-hospital affiliated “Urgent Care” facility you are screwed as, if they are not on your insurance companies list of facilities and you don’t have the money to pay out of pocket, they can refuse to see you. I could be wrong but I don’t think non-hospital affiliated private “Urgent Care” facilities are under the same rules as an ER to not deny treatment.