O Mankind, have Taqwa of Allāh ta'āla. Allāh (ﷻ) said,
“O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqūn.” (2:283)
So fasting the month of Ramadhān is one of the pillars of Islām as the Prophet (ﷺ) said,
“Islam is that you bear witness that there is no god worthy of worship except Allāh and that Muhammad is the Messenger of Allāh, and to give the Zakāt and to fast the month of Ramadhān and to perform the Hajj if you are able.”
Fasting means to refrain from the things that invalidate the fast. Refraining (with an intention) from the Hissiyah and the Ma'nawiyyah that invalidate the fast, from the second Fajr to the setting of the sun. Just as it is obligatory to fast, it is obligatory to understand its boundaries and times so that the Muslim can perform it in a way that’s desired. So he is allowed to eat, drink and consume what Allāh has made permissible for him during the nighttime from after the setting of the sun till Fajr. Then he refrains from Fajr till the setting of the sun. So it is obligatory to restrict it to these well-defined times. So the person doesn’t fast before Fajr, specifically those who stay up late at night then eat the Saḥūr and the end of their staying awake and sleep before Fajr and they don’t pray Fajr with the Muslims. And likewise, it is not permissible to continue eating and drinking after Fajr.
“And eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night).” (2:187)
And the indications of Fajr for those who don’t see it with their eyes is the Adhān of the Mu’adhin’s. So if the Mu'adhin’s is heard then he refrains from food and drink and that which invalidates the fast because the Adhān is one of the indications of Fajr. So it is obligatory on the Mu'adhin’s to determine when is Fajr and not call the Adhān before it, unless for the one who calls the Adhān before Fajr so he calls the first Adhān in order to wake the people up and then calls the second Adhān at Fajr in order for them to refrain from food and drink. As for the one who is restricted to one Adhān then it is obligatory on him to determine the times and the Mu'adhin is entrusted. So it is obligatory on him to preserve this trust and not to be careless in regards to it. If Fajr comes and he doesn’t call the Adhān and he refrains from the Adhān he might mislead the people because some Mu'adhin’s delay either because of heedlessness or being preoccupied or due to sleep or other than that. Then he calls the Adhān when he wakes up even if it is after Fajr and this is Ḥarām because he misled the people. So they ate and drank based on his Adhān after the Fajr came. So this affair is very important. Likewise when the sun sets. He doesn’t call the Adhān unless the sun has actually set and he doesn’t call the Adhān before the sunsets so as not to have those that hear him break their fast before the time of Ifṭār. This occurs often because of the negligence of the Mu'adhin’s from allowing the time go by the time of the Adhān. So he doesn’t call the Adhān, he suffices with the Adhān of the other Mu'adhin’s so as to not mislead the people.