Go and enjoy yourself. This was pre arranged and your son will be at home when your husband gets home. Anyway, hospital dates and transport change constantly for many reasons so you have no need to change your plans.
I know someone who had arranged to go out for a meal with her son and his wife, just before they were due to leave when her husband fell off a ladder and they had to call an ambulance, husband went off to A & E, wife went out for her meal! Husband had broken ribs! I think something like this is when you have to change your plans!
Another silly little ABC game - shops we have loved and lost?