ἡ Κυριακὴ ἡμέρα (Ancient Greek) comes from Ancient Greek κυριακή, from Ancient Greek Κυριακός, from Ancient Greek -ακός, from Ancient Greek -κός, from Proto-Indo-European -ḱos — -y, -ic.
ἡ Κυριακὴ ἡμέρα (Ancient Greek): the Lord's Day