One word substitutions for number of days?
Solution 1:
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.