hebdomad

From Late Latin hebdomada (“number seven; group of seven; seven days”), hebdomas (“number seven; seven days; seventh day”), from Ancient Greek ἑβδομάς (hebdomás, “group of seven, especially seven days or seven years”), from ἑπτά (heptá, “seven”) + -ᾰ́ς (-ắs, suffix forming abstract nouns of number from numerals). The word is cognate with French hebdomadaire, hebdo (“weekly periodical”), hebdomadairement (“weekly”), Portuguese hebdomadário (“weekly periodical”), Spanish hebdomadario (“weekly; weekly periodical; hebdomadary”).