When and how does one make the intention (niyyah) for fasting?

Making the intention (niyyah) is one of the conditions of fasting. A fast without intention is not valid. While it is sufficient to make the intention in the heart, expressing it verbally is recommended. Getting up for suhoor (pre-dawn meal) is also considered as making the intention.

The time for making the intention for Ramadan fasts, vows to fast on specific days, and voluntary (nafl) fasts is from sunset until just before the sun reaches its zenith (about 10 minutes before noon) the next day. However, for an intention made after dawn (imsak) to be valid, nothing must have been eaten, drunk, or done that would invalidate the fast from that time onward. Otherwise, making the intention during the day is not permissible. (Kâsânî, Bedâ’i, 2/85) For these fasts, a general intention such as “to fast tomorrow” is sufficient. However, it is more virtuous to make the intention at night and specify the fast, such as “to fast the Ramadan fast tomorrow.” A separate intention must be made for each day of Ramadan. (Mevsılî, el-İhtiyâr, 1/126-127)

For missed (qada), expiatory (kaffarah), and vow fasts not tied to a specific time, the intention must be made from sunset until the latest by the time of imsak. When making the intention for these types of fasts, it should be specified, such as “for the qada, kaffarah, or vow fast of such-and-such.”

According to the Shafi’i school, the intention for all fasts except voluntary ones must be made at night. If the intention is not made by the time of imsak, that day’s fast is not valid. For voluntary fasts, the intention can be made until just before the sun reaches its zenith. (Shirazi, al-Muhadhdhab, 1/331-332)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu