Day of rest and Sunday

Day of rest noun - A day set aside for rest.

Sunday and day of rest are semantically related. in day for resting topic. In some cases you can use "Sunday" instead a noun phrase "Day of rest".

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

Day of rest and Sunday are semantically related. You can use "Day of rest" instead a noun "Sunday".