One word substitutions for number of days?

Fortnight is used (or was used until now) for 14 days.

It is more tough with year and month, which stand for 365 or 366 and 29-31 days respectively (at least if they're solar ones). Leap year is 366 days. Quarter is 3 months and one fourth of a year.

Archaic sennight (sevennight) was just another word for week, as well as hebdomad (more facetious than archaic).

Quarantine is 40 days, though limited in its usage.

Meteorologists may call 5 days a pentad.

Catholics had octave for 8 days and still have novena for 9.