The answer
From the early days of Creation after the solar system was created, the evening and the morning constituted one day (Genesis 1). When Jonah was in the belly of the fish for three days and three nights, his fast was continuous from day to night (Jonah 1:17). St. Matthew specifically mentions that Christ fasted consecutively for forty days and forty nights while in the wilderness (Matthew 4:2). Therefore, fasting ought to generally entail a 24 hour period. Some people are accustomed to fasting from evening to evening and others are accustomed to fasting from midnight to midnight.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)