According to two recent studies, if you need to be hospitalized on a weekend, your chances of substandard, dangerous and expensive care go up. The studies suggest that, while hospitals are open on weekends, they put everything except the necessities on hold overnight, on weekends, and during holidays. If one requires a trip to the emergency room over the weekend, the patient should certainly go. However, if one is permitted to pick the time to go into the hospital for a test or procedure, the studies suggest that planning a weekday trip and avoiding being discharged over the weekend is the best plan.
You can read more about the studies here.