I'm working on a novel while trying to take into account the historical context surrounding it. It begins in 1140 AD, so the characters would use Old English, Latin, Old French, and other similar languages from that time period. It also features some vampiric characters. Over the years, these vampires have developed their own terminologies to refer to common things, such as relative time, without referring to daylight.
Today is easy to conjugate into tonight, and yesterday similarly transfers to yesternight, although that sounds somewhat odd in our language. However, tomorrow is not as easy to translate.
Morrow comes from the Old English morgen, which means morning. (Source: American Heritage Dictionary, 4th Edition) Therefore tomorrow means the next morning in its oldest variant, and the Old-English-speaking vampires would not use morrow or tomorrow, and would come up with their own words.
My question is, what words exist in a historical context that allow the speaker to refer to time periods without necessarily connoting daytime? Or, if there are none, what words would realistically have developed given the languages present?
Tomorrow is the word giving me the most trouble, but I'll also accept other answers that explain how I can refer to time without referring to the daytime. My main concern is staying in context; I don't want to make up words that have no etymological basis.
Helpful answers will give a sourced example of where the word was found and how it was used (along with what language it derives from), or an explanation of where the roots they are using to derive the new word and why it makes sense to derive the word from those roots.
I'll also accept phrases, since language is complex, and there might be no single word that does the topic justice.