I foster rescue kittens. Our normal policy for kittens is rehoming from 9 weeks, but it depends on the health and the background of the kitten.
Kittens can sometimes be quite ill after their vaccinations, to the extent that they need veterinary treatment, medication and syringe feeding so we'll hold them back until we're confident that they're properly well.
Also, depending on how well the kittens have been socialised and fed in the first few week of their lives, plus if there's been any health issues, we want them to be fully over these before rehoming, unless to experienced owners. We also neuter at 1+ kilo, if they're ready, which they're not always are.
We'd also be cautious about rehoming around Christmas. This year is unusual, but in a normal year, many people's homes are either full of strangers (to the kitten(s)) or people want to go away to other relatives or even things like Christmas trees can be an inconvenience or hazard to a kitten.
None of these help kittens with settling into their new homes, which is why we'd normally not want to rehome at Christmas, although this year might be different. If people aren't going away, aren't having visitors and are off work, it could be a good time to introduce kittens into the household. But you can't force your rescue to hand the kittens over when you want them to, but I would have thought that, if the kittens are in good health, and you're not going away or having lots of people round at Christmas, you would be able to get them soon.