What is the meaning of the idiom "a red letter day"?

An important or joyful occasion in one’s life

The idiom "a red letter day" means an important or joyful occasion in one's life. The phrase comes from the old custom of marking religious holidays in red on calendars. These days were considered to be especially important, and people would often celebrate them with special events.

The idiom is still used today to refer to any day that is especially significant or memorable. For example, you might say "My wedding day was a red letter day for me."

The other options are not correct. "A sorrowful day in one's life" would be a "black letter day." "A regretful day" would be a "day of reckoning." "A dangerous day in one's life" would be a "day of peril."