The Slowtwitch 100/100 started as the 25/25, according to memory, by longtime Reader Forum member Devashish Paul. One thing led to another, as often happens in our sport, and Dev (as he is commonly known) expanded his idea to the 50/50 and then from there.
The 100/100 celebrates our collective attempts to run 100 sessions in 100 days. Not to run consecutive days, i.e., if you run 4 days out of 5, but one of your days contains a morning and evening run, that equals 5 sessions in 5 days.
The goal is not to continue until you "fail", rather, to hit 100 sessions or, failing that, to hit 90, 80, 70 or so forth, between December 15th and March 24th.
The Challenge started on December 15th, but traditionally continues to gain registrants for a few days after its official start (because late-joiners can still log their workouts performed on prior days). As of this writing the Challenge has 651 participants, more than the 599 it had in its largest participant year.
The list of Challenge participants has everyone listed by Reader Forum user name (register for our Reader Forum and you've also registered to use our Training Log and subscribe to the Challenge). There is an attendant Reader Forum thread that chronicles our achievements and failures (as we do every year). The rules and best practices sit on the first post of that thread.