Red letter day

A red letter day is a special day that is marked with a red letter on the calendar. It's like a really important day that shouldn't be forgotten! Imagine your birthday is a red letter day because it's a super important day and everyone should remember to celebrate it. Other red letter days might be holidays like Christmas or Thanksgiving - they are very important days that we look forward to and prepare for. So if someone says "today is a red letter day!" it means it's a really special day that we should pay attention to.