hebdomad (n.)

1540s, "the number seven;" c. 1600, "a week;" from Latin hebdomad-, stem of hebdomas "seven, the seventh day; a week," from Greek hebdomas "the number seven; a period of seven (days)," from hepta "seven" (from PIE *septm; see seven) + -mos, suffix used to form ordinal numbers, cognate with Latin -mus.