Select the most appropriate meaning of the given idiom.

A red letter day

a memorable day

The most appropriate meaning of the idiom "a red letter day" is a memorable day.

The other options are incorrect.

• "A day of fun and enjoyment" is not the same as a red letter day. A day of fun and enjoyment is a happy day, but it is not necessarily a memorable day.
• "A day of horror" is not the same as a red letter day. A day of horror is a terrible day, and it is not something that people would want to remember.
• "The worst day" is not the same as a red letter day. The worst day is a day that is full of bad things, and it is not something that people would want to remember.

The idiom "a red letter day" comes from the old practice of marking important days in the calendar with red ink. These days were often religious holidays or days of celebration. So, a red letter day is a day that is special and memorable.