This has me stumped.

I have a fixed-rate CD. December's interest was $X but January's interest was $Y (less than December's). Both months are 31 days. At the end of each month the interest is moved to another account at my credit union, so interest does NOT accrue (resets back to the original principal of the CD). The difference is just under $1.

My question is this - why shouldn't the interest be exactly the same for all 31-day months and all 30-day months? Am I missing something?

Both December and January had the same number of holidays + weekend days, 10, if that matters.

Your interest from November 2019 sat in the account for an extra 2 days, as November 30 was a Saturday. No Bank transfers on Saturday. The extra interest sat in your bank until it could be moved on December 2. So you had compounding interest for two extra days in December.

That is my guess.

I think your logic/expectations are sound. Fixed rate, same period - ought to be the same interest being accrued/paid.

Maybe something changed with respect to the terms with the new year? Maybe the 'fixed rate' was only for a certain period? Maybe a calculation glitch? I like the above hunch from Aeteocles as well - maybe the period they used wasn't the calendar month.

I'd check with the bank and see what they say is behind it.

No change, it's fixed over the term of the CD. There were also some minor differences in 2019 between 31-month and 30-month periods; I used December & January as an example.

I am going to wait and see what March brings and compare that to January. If different, I will check in with the credit union.

Thanks for the help, folks.

2020 is a leap year

It's banking and money changers.... read your bible and figure out it is all evil...

Here's an old stock portfolio from about 15 years ago: