This web site has existed for 16 years and then in October 2015 we received an email with a question that has never in all those 16 years been asked.
I can see that there is an answer to the latest date that a card should be mailed.
That date is governed by the "Merry Christmas" or similar wording on the card. There is no point in wishing a person Merry Christmas after the event.
These cards should be mailed at least three working days before the 25th December, thus allowing Australia Post those two days to deliver the cards before Christmas.
Now for the earliest date to mail cards.
Wikipedia at https://en.wikipedia.org/wiki/Christmas_card states as follows:-
"Christmas cards are usually exchanged during the weeks preceding Christmas Day by many people (including non-Christians) in Western society and in Asia."
My wife and I notice that we usually receive our first Christmas Cards on or about the 1st December, for a few days no further cards arrive and then the others start to arrive, right through until Christmas Eve.
Every visitor to this site must have a date or time that they decide they really must mail those Christmas Cards.
Please send us your traditional date for writing the cards and getting them into the Post Office mail box, plus what is your reason for selecting this date.