[NoboruWataya]

"In a fortnight" is not a word, obviously we have words that refer to lengths of time greater than two days, but I think the point is that we have no single word to describe the day that is a fortnight from now.

One thing I found mildly interesting about the distinction between English and Spanish is that English has "tonight" but no single word for "last night", whereas in Spanish it's the opposite.

[superhuzza]

There is "yestereve" but it's archaic and never used. Despite not being used, totally understandable to English speakers.

[bryanrasmussen]

and overmorrow

probably not understandable to many English speakers - however archaisms might not be equally archaic in every region and dialect of English.

[bryanrasmussen]

hmm, you're right. I would probably have realized it myself in a triduum.