Since it seems like a group that already practices and values feats of fasting and endurance it's probably most likely that it commemorates a single particularl

Since it seems like a group that already practices and values feats of fasting and endurance it's probably most likely that it commemorates a single particularly notable fast attributed to a specific individual in the community's history. It's possible that that individual didn't actually succeed and it was based on a myth. But I seriously doubt some guy decided one day to try to trick his peers into doing a brutal fast that had never been done.