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What is the origin of hebdomadal?

Hebdomadal, “occurring or published every seven days; a weekly publication,” comes via the Late Latin adjective hebdomadālis “weekly,” from Greek hebdomás (inflectional stem hebdomád-) “a group of seven, a seven-day cycle, a week, a fever recurring every seven days.” Hebdomás is a derivative of the adjective hébdomos (also hébdemos) “seventh,” a complicated but regular derivative of the cardinal number heptá “seven.” Original Proto-Indo-European s is lost before a vowel in Greek, becoming h; heptá is the Greek result of Proto-Indo-European septṃ, which becomes septem in Latin, sapta in Sanskrit, secht in Old Irish, sibun in Gothic, seofon in Old English, seven in English, septynì in Lithuanian, sedm in Czech, sedem in Slovak, and siedem in Polish. Hebdomadal entered English in the first half of the 17th century.