Origin Story

National Take a Kid Mountain Biking Day has been an annual observance publicized by the International Mountain Biking Association (IMBA) since 2004, when congressional supporters passed a resolution "expressing the sense of Congress" to honor the spirit of Jacob Mock Doub on the first Saturday in October.
That's how national observances become national observances.
You can look up the speeches that supported the resolution in the archives of the Congressional Record. http://www.gpo.gov/fdsys/pkg/CREC-2004-10-05/html/CREC-2004-10-05-pt1-PgH8063-3.htm
I looked them up after reading at the bottom of a press release touting local observances that the day was meant to honor the memory of a boy who died of complications from a mountain biking accident.
And I thought, What the ...? Take a kid mountain biking because some kid died mountain biking??
But it turns out the story is deeper than that, and the memorial appropriate.
According to an account published in the Winston-Salem Journal in 2004, “Jack” Doub was a champion teenage mountain bike racer from North Carolina who died in 2002, four months after cutting his leg on a stump while jumping a big rock. The day before this relatively minor accident, he had placed second in a national competition.
The newspaper reported that the 17-year-old was so cheerful and plucky his family didn't realize an infection from the wound was spreading through his body, just that he didn't feel up to being so active. According to his obituary published on ancestry.com, at the time he died he had recently taken up photography. So basically, he was young and strong and somehow able to keep total sepsis at bay for months. But then, the week before he died, he went mountain biking again with friends and had a great time jumping things.
But afterward, at home, he complained of the flu. Emergency room physicians found no evidence of flu. His family found him dead on the floor of his room a few days later.
His father told the newspaper he didn't blame mountain biking for the accident. That Jack was very active with several sports and often injured. That he could have sustained the infection that killed him  doing anything.