How would you deal with a fast (a religious one), a day before an important performance test (a black belt test, average is 3.5-4.5 hours).
Depending on your fast (Just food? Food and Water? Tish'a B'Av I take it? I'm not Jewish but I got a friend that is and he is making this totally big deal about it which I respect.)
Simply put (if you can eat on that day and depending when the event is)
Take in High G.I carbs in the morning to get an insulin response. I would take in alot of low G.I carbs in to replenish glycogen in muscles at a steady pace up until about 2 hours till the event than nibble on some High G.I carbs to get your blood sugar up for the event. Basically what I do for wrestling if I have to make a heavy cut. There's my $0.02 not that its perfect. Christan if I'm wrong correct me cause with my luck I am.
Oh and if you can't eat that day I would consult your rabbi and see if you could take dextrose tablets (They sell them at walmart for like 5 bucks.)