Question: When does the day start according to the Islaamic law? Does it start at the time of Fajr (Dawn) Prayer or right after midnight as established by astronomers? And what is the proof for that from the Noble Qur’aan and the Sunnah (whatever is reported from the Prophet)?
Answer: The Islaamic day starts at the daybreak when the Fajr Prayer is offered. Allaah (Exalted be He) said: It is made lawful for you to have sexual relations with your wives on the night of Al-Sawm (the fast). till His (Exalted be He) Saying: So now have sexual relations with them and seek that which Allaah has ordained for you (offspring), and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Sawm (fast) till the nightfall. Moreover, the Prophet Muhammad (peace and blessings of Allaah be upon him) said: Verily, Bilaal announces the Athaan (call to Salaah) during the night, so continue eating and drinking until the second Athaan is raised by Ibn Umm Maktoom … read more here.