I'm like 99% sure I've visited every single day of the month so far but I've been getting milestone achievements the day after I should? is there any way to check if I've missed any days or get that fixed?

You can try double-checking with this thread to see if you're missing any items: Melodys Cottage List 2023 [SPOILERS]

You could also file a ticket but it's most likely going to just be that you did miss a day somewhere along the way. And since the highest achievement stops at 25 days this year and you've already got it, there's not really anything to fix there.