Last delivery day this year for Christmas is Friday 22nd December. The slots have just become available today.
I went on a few moments ago and a fair number of slots are already gone, especially morning ones! I managed to get 11-12 in the morning. I have thrown in a couple of bottles of champagne to keep my slot open, and will sort it out later on.
So, if you need Sainsbury Online for Christmas - do it now!