Day to remember and Red-letter day

Red-letter day and day to remember are semantically related In some cases you can replace phrase "Red-letter day" with "Day to remember", this phrases are similar.

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

Day to remember and red-letter day are semantically related in very special day topic. You can use "Day to remember" instead a noun phrase "Red-letter day".