Crossword Genius

Quite an occasion when the extreme left write? (3-6,3)

Ross

I believe the answer is:

red-letter day

I'm a little stuck... Click here to teach me more about this clue!

'quite an occasion when the extreme' is the definition.
I can't judge whether this definition defines the answer.

'left write?' is the wordplay.
I cannot quite see how this works, but
'left' could be 'red' (red can informally mean communist) and 'red' is found in the answer.
'write?' could be 'letter' (lettering is a kind of writing) and 'letter' is present in the answer.
The remaining letters 'day' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for red-letter day that I've seen before include "Memorably important occasion" , "Noteworthy time" , "Memorable occasion" , "Momentous date" , "Special date in the calendar" .)

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