The beginning of time of Fajr prayer is the second dawn, meaning the true dawn, and the end of time till the sunrise. There is also specific timing for Dhuhr (zuhr), Asr, Maghrib and Isha prayers. Muslims know that time of praying has come by hearing the adhan. So they prepare and go to mosques to pray the 2 rakats of al fajr prayer behind the imam and listen to a beautiful recitation of the Quran during salah.