The most common and most likely answer to this crossword clue is the 10 letter word RED-LETTER
R
E
D
-
L
E
T
T
E
R
More clues leading to the result RED-LETTER
- Memorable revolutionary character
- While speaking glanced through mail, very important
- Dog left for son is very memorable
- Soundly interpreted character as special
- Such a day is important, having reportedly studied correspondence
- Socialist landlord’s special sort of day?
- Special dog, large rather than small
- Socialist message, like the words of Jesus?
Buddy explains!
The straight part
We know that "Such a day is important" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case such a day is important = red-letter.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.