Are you in (or near to) London?
The charity which brought public attention to the plight of homeless people at Christmas time was originally known as 'Crisis at Christmas'. This charity continues to work with homeless people but now extends its activities throughout the year and so has changed its name to simply 'Crisis'. However, it remains one of the key players when it comes to providing help for the homeless through its (London based) 'Crisis Open Christmas' programme. It's currently recruiting the 5000 volunteers needed to staff this year's event:
http://www.crisis.org.uk/page.builder/crisis_o pen_christmas2006.html
If you're outside of London I'll back Fee_B77's suggestion of contacting the Salvation Army. Alternatively, the largest charity for homeless people, Shelter, will probably know of Christmas activities in your area. You can find details of your local branch here:
http://england.shelter.org.uk/home/nearyou.cfm
Chris