You'll be an online shopping pro by Christmas, flapjack!
Normally you can book 3 weeks in advance for your shop. So, when I say you do your shop and move it forwards, it goes something like this:
1st November: book slot for 22nd November and do an order that looks something like you think you'll need for Christmas.
8th Nov: move order to 29th Nov; edit if necessary.
15th Nov: move order to 6th Dec; edit if necessary.
KEEP AN EYE OUT FOR CHRISTMAS SLOTS ABOUT NOW
Some supermarkets (especially if you have a delivery pass deal with them) will release Christmas slots so you can book them with more than 3 weeks to go. These slots open at midnight (so, if the slots open for booking on 20th Nov, log in at midnight on 19th Nov) so you can then edit your order to change the date to the Christmas slot you want, then check out. No need to stay up doing a full order.
You can then edit as much as you like up until whatever the cut off time is. It's very easy! 😁