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“Business day” and “due date” for bills
It's likely that your bill always shows the 24th as the due date. Their system is programmed to maintain that consistency regardless of the day of the week that falls on. When the 24th isn't a business day it is good to error on the side of caution and use the business day prior.  It would have accepted using their system with a CC payment on the 24th because that goes through their automated system. I would hazard a guess that because your payment was submitted through your bank and arrived on the 23rd it wasn't credited because a live person would have needed to be there to do it and their live people probably don't work weekends. I do much of my bill paying online and have found it easiest to just build a couple days of fluff into the schedule to avoid problems like this. That said, if you call them and explain the situation it is likely that they will credit the late charge back to you.