The Idiom of the Day

a red-letter day

For example:

Meaning: A red-letter day is a day that is very important for some reason.

• The day Tatiana graduated from university was a red-letter day for her whole family. She was the first person in the family's history to get a university degree.
  
• Today is a fantastic day for us - a real red-letter day! We've just been blessed with our first child - a beautiful baby boy!