heb·dom·a·dar·y /hɛbˈdɒməˌdɛri/ noun, plural -dar·ies, adjective

1. Roman Catholic Church . a member of a church or monastery appointed for one week to sing the chapter Mass and lead in the recitation of the breviary.

2. hebdomadal ( def. 1 ) .

1400–50; late ME ebdomadarie < LL hebdomadārius. See hebdomad, -ary

Source: Dictionary.com.

Then the abbot gave his benediction, the hebdomadary said the prayers, all bowed toward the altar

Source: The Name of the Rose by Umberto Eco.