What is the ruling on one who vowed to fast the 13th, 14th, and 15th of every Hijri month, but sometimes forgot to fast them due to relying on the Gregorian calendar?

A vow must be fulfilled, and the days vowed must be fasted. If one does not fast them due to forgetfulness or an excuse, they must be made up, as they have become an obligation. An obligation must be made up if it is missed, whether with an excuse or otherwise.