it usual a SPIRITUAL thing..... its not just not eating. u can FAST from. plenty of things... whatever is separating you SPIRITUALLY
When Im not pregnant i fast quite a bit. Thats the way i use for weight loss.
Why would you pick a day not to eat?
Some people use it as a form of losing weight, also, sometimes they pick a day or two out of the week to fast or not eat for that whole day.
Sometimes when you get your blood drawn, you can't eat beyond a certain time. Like some women for their gestational diabetes testing, can't eat past midnight. That's all fasting is :)