I'm finding this particular skill very difficult but I'm pressing on and trying to understand how it all works - with a lot of help from these discussions and the comments/advice given by the very helpful and patient moderators. My question is, why isn't 'On nečeká na posledni den' acceptable?
"Den" is an masculine inanimate noun and its form does not change in the accusative. The form of "poslední" does not change here either, because it is modifying a masculine inanimate noun.
We considere these as "for that last day". It is useful to show that první and poslední usually do not have any demonstrative (unlike English where those are almost always required).