red-letter
adjective
red-let·ter ˈred-ˌle-tər 
: of special significance
Word History
Etymology
from the practice of marking holy days in red letters in church calendars
First Known Use
1704, in the meaning defined above
Kids Definition
red-letter
adjective
red-let·ter
ˈred-ˈle-tər
: of special importance : memorable
This was a red-letter day in my life.