It's Christmas! Or close to Christmas! And given the terrible record he has with Christmases, the Doctor has decided to spend this Christmas in the TARDIS, right outside of the normal stream of time and space. This also ought to help Astrid get over her fear of Christmas, kind of ease her into the whole "you don't always die on the holidays" thing.
HOWEVER! Before he can zip the TARDIS out beyond time and space for a very merry unChristmas, it must be properly decorated. With EVERYTHING. Lights, and tinsel, and more lights, and more tinsel, and a tree, and holly, and boughs, and presents, and stockings, and nutcrackers, and there will be so much DECKING. The TARDIS will be DECKED OUT. Like woah.
And it's always more fun to decorate with friends. It makes things go quicker, and you can play in all the boxes and the wrapping paper and argue over where the ornaments should go.
SO! There's a Christmas decorating party in Ten's TARDIS and everyone's invited! Anyone he knows, anyone who he hasn't properly yet met but might wish to make his acquaintance or the acquaintance of anyone else who might show up, whatever. It'll be assumed he's parked it somewhere with some kind of mild Anti-Violence Field, so there'll be no chance of sabotaging the TARDIS or hurting anyone
Threadjacking and hopping is go, unless folks lock threads; and threads can take place at different times, so characters can come back to the party more than once. AND. Folks can thread in any combination, even without Watch!Ten being involved. It's a party, the doors are open, mingling will happen, and he's a terrible, terrible host who may be in a back room
( Feliz Navidad! Mi TARDIS es su TARDIS! )