In the religions of ancient peoples and civilizations, fasting was a practice to prepare persons, especially priests and priestesses, to approach the deities. Fasting for special purposes or before or during special sacred times remains a characteristic of major religions of the world. Judaism, Christianity, and Islam emphasize fasting during certain periods. Judaism, which developed many dietary laws and customs, observes several annual fast days, primarily on days of penitence (such as Yom Kippur, the Day of Atonement) or mourning. Christianity, especially Roman Catholicism and Eastern Orthodoxy, has observed a 40-day fast period during Lent, a spring period of penitence before Easter, and during Advent, a penitential period before Christmas. Among Roman Catholics the observance has been modified since the Second Vatican Council to allow greater individual choice, with mandatory fasting only on Ash Wednesday and Good Friday during Lent. Protestant churches generally leave the decision to fast to individual church members. The month of Ramadan in Islam is a period of penitence and total fasting from dawn to dusk. Sawm (the Arabic word for fasting) is one of the pillars of Islam (the five basic tenets of the Muslim religion), and although it is most commonly understood as the obligation to fast during Ramadan, it is more broadly interpreted as the obligation to refrain between dawn and dusk from food, drink, sexual activity, and all forms of immoral behaviour, including impure or unkind thoughts.
(Encyclopedia Britannica, topics of fasting and Ramadan)