The shelters are run by some 3,000 volunteers, many with specialist skills and experience. Far from offering homeless people just a roof and a bed, they provide a range of 21 separate services - including doctors, dentists, chiropodists, hairdressers, free food and clothes, legal and housing advice, massage, and counselling.
"Most of these people don't have access to any of these services for a whole year," says Bob Evans, who manages all the resources donated for Christmas week. "We call it giving them a complete MOT."
It's a huge logistical effort. Last year Crisis served 6,000 meals and gave out 2,000 blankets, 3,500 towels, and more than 1,000 pillows. This year the Dome will house a "be inspired" learning zone, where people can get IT training, use an internet cafe and a book club, and attend taster sessions in plumbing, art and basic numeracy.
"Lots of them will want to carry on learning," says Sarah Walker, a member of the Crisis Open Christmas fundraising team. "It's like giving them a passport they can take into the new year."
As well as those who sleep rough, Crisis says there are about 380,000 hidden homeless people in Britain, living in hostels, temporary bed and breakfast accommodation or derelict buildings, or on the floors of friends and family. There are about 100,000 hidden homeless people in London.
If you are interested in volunteering contact Leah Jeffries, tel: 020 7426 3853 or go to www.crisis.org.uk