Thanks much appreciated. I did telephone Alliant on this and did my level best to obtain a reliable answer. Was not permitted to speak with back office, but the supervisor helping me did speak with them:
Claim is that Sunday is a good, legitimate paying day at Alliant. Claim is you make a payment on a Sunday (Sunday night, actually, is when the money is dispatched), it will be processed that day and arrive next business day.
Was motivated to make this inquiry based on situation I face (pretty much) right now. I have a CC payment that needs to arrive on the ninth. The eighth is a bank holiday (sucks). It’s a lot of money. I wanted to avoid making the payment tomorrow (Friday) if I could (lost interest).
Am gonna go the Sunday route, Sunday the seventh. Asked about doing same on the eighth. No dice! They will not process any payments on the holiday itself. But there will be bill payer service on Sunday, like always. Hope this works out.
ETA
BTW and strictly FWIW, in the course of my inquiry the supervisor volunteered (I did not ask) that there is no Alliant bill payer service on Saturday. You schedule a payment on Saturday, it will be paid next day, on Sunday.