It was partly influenced by the Mithras worshippers, which were very numerous in the early days of Christianity. Mithras was the god of the sun, and Sun-day was his day. (Mithraism also contributed more to Christianity: the birth on Dec.25, baptism in blood, birth witnessed by shepherds, etc.) The Romans also called it "the Lord's day," and it was observed as special. Early Chiristians
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