1

Book directly at the webpages of the Swedish railrays, but you can't book until 90 days before departure. Only 3-bed cabins are available, they'll cost about SEK 1800 for two adults. As you are traveling just before Christmas, you're likely to get a third person in the cabin unless you oay for the third bed as well.

In addition to SAS, FlyNordic has regular flights between Stockholm and Kiruna.

2

All fligts trains etc will be costly the day before Christmas where "everybody" are going home on family visits and the last business travellers are returning home at the same time.

3

i don't know what you are planning to do in kiruna but because winter is high season in lapland, i think you should book accomodation right away too.

4

at this booking site there dont seem to be any sleeper trains on the 23rd
FYI the train to Kiruna is run by Connex (www.connex.se) not SJ

5

Actually, this Stockholm to Kiruna service is not run by Swedish Railways but by Veolia. See www.veolia-transport.se. Connex, referred to by #4 above, is the old name for Veolia. I can highly recommend the service. It operates a simple-to-understand fixed fare system. Stockholm to Kiruna one way is always 399 SEK (ie. about €43) per person. Yes, the train has sleeping cars with berths fo two persons. They are very comfortable,. The supplement for that is 499 SEK (€54). There is a good restaurant car on the train. Online bookings for the Christmas period have not yet opened, but if you call +46 771 26 00 00, you can check all details, perhaps even reserve now, and at least ascertain when you might be able to book online. I'd rate this as one of Europe's great train journeys. You can read a six page feature on the journey, with lots of details about the train in the July 2007 issue of hidden europe magazine. See www.hiddeneurope.co.uk.
I do hope this helps.
Nicky