'antepaschal' originates from Latin and Medieval Latin elements: Latin 'ante' meaning 'before' and Medieval Latin 'paschalis' (from Latinized Greek/Aramaic 'pascha') referring to 'Passover/Easter'.

'antepaschal' developed from Medieval Latin 'antepaschalis' (formed from 'ante-' + 'paschalis') into English in ecclesiastical and liturgical usage, retaining the combined sense 'before Pascha (Easter)'.

Initially it specifically indicated things 'before Pascha/Passover' in liturgical contexts; it has kept this specialized meaning and is still used chiefly in church or historical contexts to mean 'occurring before Easter'.