A person can make niyyat every night of the Holy month of Ramazan that he would be fasting on the following day, and it is better to make niyyat on the first night of the month of Ramazan that he would be fasting throughout the month. The last time for making niyyat to observe a fast is moments before Azan of Fajr prayers. As for ‘Mustahab’ fast one can make its niyyat at any time in the day, even moments before Maghrib provided he has not committed any such act which invalidate the fast. If a person sleeps before Azan for Fajr prayers in Ramazan or any other day fixed for an obligatory fast without making a niyyat, and wakes up before Zohr to make a niyyat of fast, his fast will be in order. But if he wakes up after Zohr, as a precaution (Ehtiyaat), he should continue with the abstinence with the niyyat of Qurbat and then observe its ‘qaza’ also.   _{Your Comment}