A book or manual that contains information about religious festivals, feast days, and their dates and significance.
From Greek 'heorte' (festival) combined with '-logion' (a collection or book about something), used in ecclesiastical contexts to describe religious calendar texts.
Imagine a medieval priest consulting the heortologion like we'd check Wikipedia—these books were the original 'holiday management systems' for churches, tracking centuries of special days and their meanings!
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