When and how should one make intention for fasting?

Intention (niyyah) is one of the conditions of fasting. Fasting without intention is not valid. While intending only by heart is enough, expressing it verbally is mandub. Waking up for suhoor is also considered as an intention.

The time of making inteniton for Ramadan fasting and nafl fasts is from the sunset until when the sun is at its zenith the next day. But, for the validity of the intention after imsak, nothing should have been eaten and drunk and no action contrary to fasting should have been conducted. Otherwise, intention after fajr does not become permissible (Kasani, Bada’i, II, 85). An absolute intention as “to observe fasting tomorrow” is sufficient for such fasts. Yet, it is more virtuous to make intention at night and determine the fasting as “to fast tomorrow.” One needs to make intention for each Ramadan day separately (Mawsili, al-Ikhtiyar, I. 397,400)

One needs to make intention for fastings such as qada, kaffarah and votive fastings regardless of a certain time from sunset until fajr. While making intention for such fasts, one needs to clarify to which qada, kaffarah or votive fasting s/he is intending.

According to the Shafi’i school, one needs to make intention at night for all kinds of fasts except for nafl. If one has not made intention until fajr time, fasting of that day is not valid. One may make intention for a nafl fast until when the sun is at its zenith (Shirazi, al-Muhadhdhab, I, 331-332).

Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS

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