My payment was due on a Sunday and I understood that meant I had until the next business day (Monday) to pay. I went online to my card issuer's website and made a payment on Monday, but I was charged a late fee. Can they do that?
Yes. If your payment is due on a day on which mail is not delivered (such as a Sunday) and you mail your payment, you cannot be charged a late fee if your payment arrives on the next business day.
However, if you pay online you must make your payment on the day it was due even if that day is a Sunday or holiday.