HEBDOMADARIUS, or HERDOMADIUS, a member of a chapter or convent, whose week it is to officiate in the choir, to rehearse the anthems and prayers, and to perform the usual functions which the superiors perform at solemn feasts, Hebdomadary and other extraordinary occasions. The word is formed of the Greek ἑβδομάς, which signifies the number seven; of ἕβδος, seven.

The hebdomadary generally collates to the benefices which become vacant during his week; though it is usually looked upon as an abuse.

In cathedrals, the hebdomadary was a canon or prebendary, who had the peculiar care of the choir, and the inspection of the officers for his week.

In monasteries, the hebdomadary is he who waits at table for a week, or other stated period; directs and assists the cook, &c.