On the first Sunday of every month (with some exceptions), the LDS Church holds a 'fast.' To fast is to go without something for a period of time. In the Mormon Church, "fasting" typically refers to giving up food and water for a 24-hour period (or for 2 consecutive meals). There is a principle of sacrifice involved in fasting that allows us to have a more clear communication with God.   Even in the Mormon faith, fasting may be up to personal interpretation. Mimi mentions a friend of hers who won't chew gum while fasting since he feels like doing so goes against his fast.   Justin talks about a time during his mission where he fasted too frequently and ended up less useful than if he just would have fasted once a month.   Fasting isn't only practiced by Mormons. Many religions practice fasting, though the practice itself might differ religion to religion. In this video, the 3 Mormons mention a list of religions who also observe a 'fast.'