Would you be doing this to lose weight and if so would you be referring to the 5:2 intermittent fasting regime? If so, several of my friends have been successful and would consider it worth doing, my wife on the other hand actually gained weight and I'm guessing that was because she made up for the two 500 calories days by eating considerably more on the following day or it could be that like weight lifters who fast, she's increased her muscle mass and decreased her fat mass. Intermittent fasting might sound like a misnomer when you're consuming 500 calories on the two fasting days, but that's what it's called.
Apologies if this question seems a bit flippant, isn't fasting from 8pm to 12 am called skipping breakfast? As far as exercise is concerned, my friends avoided exercise like squash and singles tennis on the 500 calorie day.