Ours were nearly 8 and 2.5 when they moved in.
We had a 4 week intro period which was way too long for youngest. it did include some blocks of overnight stays so really it was more like 3 weeks.
Personally, and I only have experience of ours, I would think the plan shouldn't include a bridge over Christmas. So it should either be move in before Christmas, or start intros in January. Maybe it depends how long they have been with FC? Were they there last Christmas?
Don't underestimate how absolutely exhausting and overwhelming it may be. If they are with you for Christmas you may need to hunker down and keep things low key so as not to overwhelm both them and you.
Even though ours had been with us for half a year, we still found buying the 'right' presents hard for the elder one difficult, so a variety of lower cost ones might be better than a big one that might not hit the mark.
We had discussions in the run up as to what would happen Christmas day which was lucky as which presents Father Christmas brings was different to their 2 xmases in foster care.
I think 6 could be a hard age - old enough to understand some, but not really old enough iyswim? Throwing Christmas in too could be hard. Our 8yo was very tearful for a few weeks in the evenings as she processed that yes she and her sister were now safe, but that she wasn't seeing birth mum anymore (and why couldn't birth mum come and live with us).
otoh, moving in before will mean that next year it will be 'familiar' rather than a 'first'.
Best wishes whatever is decided.