The slots open up for booking 3 weeks in advance. So once Christmas week slots - ie., 18th December onwards open up, they get booked up really quickly. So count three weeks back from that week (the week running up to Christmas) - and as soon as the slots are bookable, do your shop and choose the delivery day you want.
I don't ever have mine delivered on Christmas Eve, btw - I leave myself a day or so in case there is stuff missing. One of my friends had a Christmas Eve delivery one year and had no turkey, no bacon, no potatoes, no sprouts, no crackers, no Quality Street (she was most peed off about the Quality Street, I think). She ended up trawling round the shops with two children in tow, in the dark, on Christmas Eve. Not good.
Delivery charges are the same in the week, iirc - maybe more for Christmas Eve/New Years' Eve (but this year it's a Sunday so would be more anyway).