As to Ramadan fast and that to be observed in a specific day, one should intend fasting the night before — the early night up to morning adhān and for non-specific fasts, e.g. qaḍā’ fasts and absolute nadhr from the early previous night until…
1. Fasting in Islam is to refrain from eating, drinking, etc. — which will be discussed later in details — the whole day (from fajr until maghrib) in order to obey the Allah's order.