as has already been said, overmorrow is already mostly a thing and is completely cromulent, and i propose taking the swedish "förrgår" and bringing it in as something like "foremorrow" which sounds reasonably cromulent to my ear, might confuse people a little bit but the "fore" bit is a pretty big hint as to what it means.
Also a fun one "Estrenar", which can mean something like "try for the first time". So you might say "I tried out my bike for the first time the day before yesterday" in English, you could simply say "Estrené mi bicicleta antier" in Spanish
"Overmorrow" is the word for the day after tomorrow, and "ereyesterday" is the word for the day before yesterday, though both are obviously archaic and not really used (you perhaps might see them in fiction or historical work, though).