Before -- long before -- flash mobs, there was Tuba Christmas. For almost 40 years, hundreds of tuba players have gathered annually in downtown Denver for a holiday concert.  They get one rehearsal, a couple of hours before the show. They buy their own sheet music -- easy for the newbies, harder parts for the experts. Generations of families play together and there are two conductors so everybody can see somebody who's keeping time. Director Bill Clark tells us more about Tuba Christmas...

This year's free concert is at 1 p.m. Sunday at Skyline Park in downtown Denver. 

[Photos: James Jacobs]