Advice of Scholars - KINDS OF FAST

There are eight kinds of fast:

The fasts that are fard. Fard fasts also have two kinds: the one which is performed at a certain time, fasting during Ramadân-i-sherîf.
The fast that is fard and yet which is not performed at a certain time. Examples of this are the fasts of qadâ and kaffârat. But the fast of kaffârat is fard-i-amalî. That is, he who denies it does not become a disbeliever.
The fast that is wâjib and which is performed at a certain time, too, such as vowing to fast on a certain day or on certain days.
The fast which is performed at haphazard times.
The fast that is sunna, e. g. fasting on the ninth and tenth days of Muharram.
The fast that is mustahab, examples of which are fasting on the thirteenth, fourteenth and fifteenth days of every Arabic month, fasting only on Fridays, fasting on the day of ’Arafa, which is the day previous to the ’Iyd of Qurbân (Ad’ha). It is also said (by some savants) that it is makrûh to fast only on Fridays. A person who wants to fast on Friday had better fast on Thursday and Saturday, too. For it is better not to do something which is said to be sunnat or makrûh.
The fast that is harâm. It is harâm to fast on the first day of the ’Iyd of Fitra and on any of all four days of the ’Iyd of Qurbân.
The fast that is makrûh: to fast only on the tenth day of Muharram, only on Saturdays, on the days of Nawruz and Mihrijan, [which are the twentieth days of March and September], to fast every day throughout the year, and to fast without talking at all.