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

an important day

The correct answer is ☀ an important day.

The idiom "red letter day" means a day that is special or important. The phrase comes from the old custom of marking religious holidays in red ink on calendars. These days were considered to be more important than other days, and people would often celebrate them with special activities.

Today, the phrase "red letter day" is often used to refer to any day that is especially happy or memorable. For example, you might say that your wedding day was a red letter day for you.

The other options are incorrect. "A difficult day" and "a difficult situation" do not have the same positive connotation as "red letter day." And "a natural disaster" is not typically considered to be an important day.