Is relying on the calendar more correct for starting and ending the fast, or relying on sighting with the naked eye? And what is the ruling for one who fasts and breaks their fast based on sighting with the naked eye?

Fasting entails abstaining (imsak (definition)) from the emergence of true dawn (Al-Fajr Al-Sadiq) and breaking the fast (iftar (definition)) at sunset. This is based on the Almighty's saying: "And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the night [i.e., sunset]," and the Hadith (definition): "When the night comes from this side, and the day departs from this side, and the sun has set, then the fasting person has broken his fast."

It is permissible to rely on calendars that are based on observing these signs, especially for those who find it difficult to see the dawn or sunset. As for those who can see the signs with the naked eye, they should act according to what they have seen. And for those who cannot, they should rely on the calendars established by trustworthy scholars. If one cannot find calendars and is unable to see the signs, then one should not break the fast except with certainty or strong presumption of sunset, and should eat at night until certain or strongly presumes the emergence of true dawn.