The most common and most likely answer to this crossword clue is the 6 plus 3 letter word CANADA DAY
I've seen this crossword clue in The Times
C
A
N
A
D
A
D
A
Y
Buddy explains!
The straight part
We know that "date of national importance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case date of national importance = canada day.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.