Some examples are easier than others, like the mentioned 12/25. 12/12 is Saturday -> 25-12 is 13 more days -> same day as 1 before 12/12 -> Friday.

I don't know what's wrong with me, but I can't do mental calculations with dates because I always think I'm off-by-one, no matter what.

I prefer to "move vertically" in the month. Knowing that 1=8=15=22=29, using prlin's example for December 25 (sibling comment), I'd go like:

"12 is Saturday, 15 is Tuesday, 22 is also Tuesday, so 25 is Friday."

So, starting from key date (12), moving to "the 1 column" (15), moving up or down to the closest/easiest date (22), and then finishing from there.

Slower for sure, but easier for me to not lose track. And now having a known day in every month thanks to rmunn's mnemonic, is going to be a huge help.

Just to clarify, I didn't invent this. I don't think you were attributing it to me when you said "rmunn's mnemonic", I think you meant "the mnemonic that rmunn posted". But just in case anyone thinks I invented this, I didn't. I don't remember where I got it from, it was twenty-plus years ago that I learned this mnemonic, but I didn't invent it.