so, the third panel there: Jake doesn't forget what day it is, he just has no idea how to express "May 28th" in Na'vi, so he switches to English.

The Na'vi of course don't traditionally use the 12-month calendar we have here on Earth. However, there is a way to express such dates in the language...mostly, I suspect, for the convenience of us nerdy fans who want to talk about our humany matters in Na'vi lol, but I believe the in-universe explanation is that they learned about it from Grace and only really use the terms if they need to talk in sky people timetables for whatever reason.

Because it's a borrowed concept, the terms are quite literal descriptions: "month" is vospxì, which is short for vosìpxì zìsìtä, literally meaning "one-twelfth of a year". To name specific months, you just add their number: May is therefore vospxìmrr, essentially "one-twelfth-of-the-year number five".

As for the day, that's actually a little trickier because Na'vi counts in base 8, not base 10. Instead of thinking of this number as two-tens-plus-eight like we do, they'd think of it as three-eights-plus-four: pxevosìng. Unless you're very good at math (which I am not lol), these decimal-to-octal conversions can be really hard to keep track of/calculate on the fly even before you try to start applying the Na'vi vocabulary!

All that to say, "May 28th" in Na'vi would be something along the lines of trr apxevosìve vospxìmrrä - 28th day of May, or most literally, "28th day of one-twelfth-of-the-year number 5".

...yeah, I don't blame Jake for just switching back to English for this 😅