WAJIB CATEGORY OF FASTING

The word 'Wajib' here means 'essential' or 'necessary'. It does not refer to the technical meaning of the Fiqhi term, Wajib, the definition of which appears at the end of this book under the title:
Necessary Technical Terms of Fiqh.
The category of Wajib Saum includes all forms of compulsory fasts -both Fardh and Wajib.
The Wajib category is sub-divided into two classes of Saum, viz.

Saum related to time.
Saum which devolves upon one as an obligation.

(a) SAUM RELATED TO TIME
This type of Saum comes into effect when the occasion or time for its observance arrives. This type of Saum comprises:
The Saum of Ramadhan.
The Saum of Nathrul Muayyan.
Nathrul Muayyan refers to a vow in which it was stipulated that a particular day/s will be fasted upon materialization of one's wish or dua.

NIYYAT FOR THIS FIRST WAJIB CATEGORY
It is perferable to make the Niyyat for this Wajib category of Saum during the night, i.e. prior to the entry of Subah Sadiq when the fast starts.
If the Niyyat was not made during the night, the Saum will be valid if the Niyyat is made approximately one hour before Zawwal. One hour before Zawwal is an approximate time taken as a precautionary measure. The exact time limit for the validity of the Niyyat for Saum is before the time of Nisfun-Nahar, i.e. mid-day in terms of the Shariah.
Nisfun-Nahar is determined by dividing by two the time duration from Subah Sadiq to sunset, and adding the result to Subah Sadiq time.
Example:
Subah Sadiq 5:30 a. m.
Sunset 5:35 p. m.
Time duration from Subah Sadiq to Sunset:
12 hrs. 5 mins divided by 2 =6 hrs. 2 mins. Add this result to Subah Sadiq:
5:30 + 6:02 = 11.32 a.m. = Nisfun Nahar
If Niyyat is made for the Saum before 11:32 a.m. (in this example), the Wajib-Saum will be valid.
It is not essential that any particular Niyyat formula be recited. Niyyat merely means to intend that one will be fasting, or is fasting. The Niyyat could also be made verbally in any language, e.g. one may say:
'O Allah! Tomorrow I shall be fasting for You'. Or one may recite in Arabic e.g. "Nawaitu bi saum-il yauma min shahar-i-Ramadhan".
"I intend to fast for this day in the month of Ramadhan (for the sake of Allah only)".
Remaining without food and drink the whole day will not render such abstention a Saum if no Niyyat was made.
During the month of Ramadhan only Niyyat of the existing Ramadhan Saum will be valid. Even if a Niyyat for any other Saum, is made during Ramadhan, then too only the Saum, of the existing Ramadhan will be discharged and not the Saurn for which Niyyat was made.

(b) SAUM WHICH DEVOLVES AS AN OBLIGATION
This is the second category of Wajib Saum (stated earlier). Examples of Saum in this category are:
(1.) Qadha of Ramadhan
(ii) Nathrul Mutlaq which is a vow made to fast any number of days without stipulating any particular day or date in the vow.
Kaffarat.
Saum, of Zihar

NIYYAT FOR THIS SECOND CATEGORY OF WAJIB SAUM
This type of fast will be valid only if niyyat was made during the night, i.e. before Subah Sadiq.
If niyyat for this type of Saum was made after Subah Sadiq, then the Wajib Saum of this category, will not be discharged, and the Saum thus kept will be a Nafl fast.