It's Apple Card payment due date for most, and an outage is causing problems
Apple has acknowledged that an Apple Cardoutage could be causing problems for users on the day that the card's bills are due.

Apple Card
Holders of the Apple Card began reporting issues with accessing their Apple Card management options on Monday morning. Users report being met with an "account services unavailable" dialog box when attempting to pay their bills.
According to Apple's system status page, an issue with Apple Card services may cause issues when adding Apple Card to Wallet, paying a bill, or applying for the Goldman Sachs-backed credit card.
The issue has been ongoing since 9:26 a.m. Eastern Time on Jan. 31, Apple's system status page indicates. There's no word on when the issue might be resolved. Apple Card payments are due on the last day of the month.
Although Apple says the bug affects "some users," reports continue to roll in on social media platforms like Twitter.
An AppleInsider staffer also confirmed running into the error message when accessing their Apple Card.
AppleInsider will update this piece when the issue is resolved.
Read on AppleInsider

Apple Card
Holders of the Apple Card began reporting issues with accessing their Apple Card management options on Monday morning. Users report being met with an "account services unavailable" dialog box when attempting to pay their bills.
According to Apple's system status page, an issue with Apple Card services may cause issues when adding Apple Card to Wallet, paying a bill, or applying for the Goldman Sachs-backed credit card.
The issue has been ongoing since 9:26 a.m. Eastern Time on Jan. 31, Apple's system status page indicates. There's no word on when the issue might be resolved. Apple Card payments are due on the last day of the month.
Although Apple says the bug affects "some users," reports continue to roll in on social media platforms like Twitter.
@AppleCard logging in to pay doesn't seem to work either on phone or online. Help! pic.twitter.com/k4TCuDZNu8
-- Jim Helisek (@helisekjames)
It looks like there could be an issue with Apple Card and making a Payment right now. Tried myself and had others comment on it where you try to pay, but it says unavailable. pic.twitter.com/qDlI4kdpet
-- Aaron Zollo (@zollotech)
An AppleInsider staffer also confirmed running into the error message when accessing their Apple Card.
AppleInsider will update this piece when the issue is resolved.
Read on AppleInsider

Comments
I’m assuming that any interest accrued because of these payment issues will be refunded, but it’s still a pain to deal with.
The problem with trying to pay early is that, at least for me, I don't get the option to pay with Apple cash (up to what I have) and having the balance after that paid from my checking account. Any time I try and pay befoee the deadline it only wants to take from my checking account. But paying on the due date will take from Apple cash first and balance from checking.