Personally they are only a few special occasions that I will fast for, one being after someone dies and I prepare for the Ritual of the Passing.
I do however commit to ritual baths before I do most of my workings, but fasting is actually depriving the body of nutrient and nourishment lowering the bodies energy. But that is how I view it. It all comes down to preference, beliefs, and balance meaning which ever works best for you, is the way to go.
If it makes any sense however, I do refrain from eating unhealthily before ritual work, I usually partake in a salad, or some fruit. I stay away from most meats, fried foods, confections, and of course any junk food.
Blessings and Peace be with you,