From the given options select the one that rightly explains the given expression:

A red letter day

Significant and special day

The idiom "a red letter day" most clearly explains a significant and special day. The idiom comes from the literal meaning of a day marked in red ink in a calendar, which was traditionally done to mark religious holidays. In the same way, when someone says that a day is "a red letter day," they are saying that it is a day that is special and worth celebrating.

The other options are not as accurate. When the sun looks red means that the sun is low in the sky and is reflecting off of dust or pollution. Hectic and boring day means that the day is very busy and not very enjoyable. Holiday means a day on which people do not work and often celebrate something.