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 about this clue!

Here is my best explanation:

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

'left write?' is the wordplay.
I cannot quite see how this works, but
'left' could be 'red' (I've seen this in other clues) and 'red' is present in the answer.
'write?' could be 'letter' (lettering is a kind of writing) and 'letter' is located 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 clue (or it may be nonsense).

Can you help me to learn more?