"until" is used for an activity that continues until a specific point. e.g. I was enjoying myself at the party until my horrible neighbour arrived. (=at the moment of his arrival, my enjoyment stopped) In your example, we are talking about specific occasions that have a start and an end. The enjoyment is not continuous, but for example, on one day, then another day, then a different day (perhaps a week later).

Why is until unacceptable in this case? To me, when you say “until,” it means that the people enjoy themselves (Eating? Drinking? Partying?) up to the exact moment that fasting begins, with no break in between. Some people might do that, but it’s probably more accurate to say that they enjoy themselves sometime before the fast. “Until” is not wrong, but “before” makes the timeframe a little less precise.