Lord's day and Rest day

Lord's day noun - First day of the week; observed as a day of rest and worship by most Christians.

Rest day and Lord's day are semantically related. In some cases you can use "Rest day" instead a noun phrase "Lord's day".

Rest day noun - A day set aside for rest.

Lord's day and rest day are semantically related. Sometimes you can replace phrase "Lord's day" with "Rest day", this phrases are similar.