Is it necessary to renew the intention each day for every day of an obligatory fast? And what is the ruling on the fast of someone who fasted their whole life without a renewed intention, sufficing with the intention to fast the entire month?

To fulfill the intention (niyyah) required for fasting, it suffices to simply resolve to fast at any point during the night, such as when the thought crosses the mind of the fasting person, or when partaking in the pre-dawn meal (suhoor) with the intention to fast. The Hanafis, Shafi'is, and Hanbalis, in a relied-upon opinion, hold that each day of fasting requires an independent intention. In contrast, the Malikis and some Hanbalis are of the view that for continuous fasting (like Ramadan), a single intention at the beginning is sufficient for the entire period.