What helped me is that “the other day” is very common English expression, meaning something that happened few days ago.
“The other month” is not so, it does not roll of my tongue, so it feel awkward, and I switch to “last month”.
I bet the fact that you’ve made a post about it will help you remember.
Another funny case for wrong translation of 先日 from anime, spoilers for Boruto
I hate Boruto but they were making anniversary arc with time travel to Naruto time, so I’ve watched it.
I still try to avoid spoilers, so all is not specific. The character used 先日, and English translation was “Yesterday, this particular troublesome event happened”. And all Naruto fans (including myself) were like WHAT, it could not have been yesterday, since such and such character had to be in that pace at that time and so on and so on, referring to events in Naruto. Simple explanation - it was “the other day”, maybe a week or two ago, not yesterday. Wrong translation. 