Red-letter day and Religious festival

Red-letter day noun - A memorably happy or noteworthy day (from the custom of marking holy days in red on church calendars).

Religious festival and red-letter day are semantically related. In some cases you can use "Religious festival" instead a noun phrase "Red-letter day".

Religious festival noun - A festival having religious significance.

Red-letter day and religious festival are semantically related. You can use "Red-letter day" instead a noun phrase "Religious festival".