主日（拉丁語：dies dominica，源自通用希腊语：he kuriake hemera）即主的日子，亦称礼拜日，在基督宗教傳統中，即是星期日，在這天，基督徒會舉行宗教儀式。
The first day of the week in many religious traditions, and the seventh day of the week in systems using the ISO 8601 standard; the Christian Sabbath; the Lord's Day; it follows Saturday and precedes Monday.