Niyyat for Fasting

It is not necessary for a person to pass the niyyat for fasting through his mind or to say that he would be fasting on the following day. In fact, it is sufficient for him to decide that in obedience to the command of Allah he will not perform from the time of Adhan for Fajr prayers up to Maghrib, any act which may invalidate the fast. And in order to ensure that he has been fasting throughout this time he should begin abstaining earlier than the Adhan for Fajr prayers, and continue to refrain for some time after sunset from acts which invalidate.

 A person can make niyyat every night of the holy month of Ramadhan that he would be fasting on the following day, and it is better to make niyyat on the first night of Ramadhan that he would fast throughout that month. 

The last time for making niyyat to observe a fast of Ramadhan for a conscious person, is moments before Adhan of Fajr prayers. This means he must be intent upon fasting at that time, even if he later became heedless of his intention due to sleep etc.

 As for Mustahab fast one can make its niyyat at any time in the day, even moments before Maghrib - provided he has not committed an

Adhan for Fajr prayers in Ramadhan or any other day fixed for an obligatory fast without making a niyyat, and wakes up before Zuhr to make a niyyat of fast, his fast will be in order. But if he wakes up after Zuhr, as a precaution, he should continue with the abstinence with the niyyat of Qurbat and then give its qadha also.
t which invalidates the fast.