(Позавчора (Pozawczora)/Przedwczoraj (PL) - "the day before yesterday", I've always wondered how many other indo-european languages have a separate word for this. Very handy.
In Hebrew (my native) there's a word for that, but it's not an indo-european language. also, I think in German you can say "vorgestern".
Also not an Indo-European language, but Chinese also has a two-character combination that basically equates to a single word, 前天 qíantīan. I'm beginning to think English is in the minority on this!
Incidentally, my Russian ex-boyfriend once asked me how to say 'the day before yesterday' in English. He was absolutely convinced that it was simply 'before yesterday,' a pattern which, as we can now see, fits in so many other languages. He was so amazed that English required so many words to express the concept that he refused to believe me - a native speaker! - that this is how we say it in English until his linguist father confirmed it!