All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There are many Prophetic narrations which clarify the days that are desirable for a Muslim to fast; they are as follows:
Six days from the month of Shawwaal: Abu Ayyoob Al-‘Ansaari narrated that the Prophet said: “He who fasts Ramadan and follows it with six days of Shawwaal, this is as if he fasted for the whole year.” [Ahmad, Abu Daawood and At-Tirmithi]
The first ten days of Thul-Hijjah: Hafsah said: “The Prophet never refrained from doing four (things): Fasting on ‘Ashoora’ day, fasting the first ten days of the month of Thul-Hijjah, fasting three days of every month, and (praying) the two Rak'ahs before the Fajr prayer.” [Ahmad and An-Nasaa'i ] The first ten days of Thul-Hijjah actually means the first nine days, because the tenth day is the ‘Eed day and fasting it is forbidden.
‘Arafah day: This is for the people who are not performing Hajj. Abu Qataadah narrated that the Prophet said: “Fasting on the day of ‘Arafah removes the sins of two years: the previous and the coming year, and fasting on the day of ‘Ashooraa’ wipes out the sins of the previous year.” [Muslim, Abu Daawood, An-Nasaa'i and Ibn Maajah] Also, Abu Hurayrah narrated that the Prophet prohibited the fasting of the day of ‘Arafah in ‘Arafah (i.e. for the pilgrims).” [Ahmad and Ibn Maajah]
Fasting during the month of Muharram: Abu Hurayrah narrated that the Prophet was asked: “Which Fast is best after the month of Ramadan? He replied: ‘‘The month of Allaah Al-Muharram.” Among which is ‘Ashooraa’ day, which is the tenth day of the month of Muharram, and the ninth day of the same month should also be fasted because Ibn ‘Abbaas said: “When the Prophet fasted ‘Ashooraa’ day and instructed others to fast it, they told him that it is a day which the Jews and the Christians glorify. Thereupon, he said: ‘‘We will fast the ninth next year, Allaah willing.’’ [Ahmad and Muslim] However, the Prophet died before the advent of the following year. The excellence of this day is mentioned above in the narration from Abu Qataadah .
(Some) days of the month of Sha'baan: Aa’ishah said: “The Prophet did not fast in any month as much as he did in Sha’baan. He used to fast (all its days) except few.” [Al-Bukhari and Muslim]
Three days of every month: Abu Hurayrah and Abu Tharr said: “The Prophet advised us never to quit (praying) the two Rak'ahs of Ad-Dhuha, and the Witr (prayer) before going to bed (at night); and to fast three days of every month. It is desirable if these days are the thirteenth, fourteenth and fifteenth days (of each lunar month).” [Muslim]
Mondays and Thursdays: ‘Aa’ishah said: “The Prophet used to seek to fast Mondays and Thursdays.” [Ibn Maajah and An-Nasaa’i]
Therefore, there are many preferred days to fast but if a Muslim restricts himself to fasting some preferred days and adhere to them, this is better for him than fasting all these days and then quitting them all because “when the Prophet used to observe any good deed, he would never quit it.” [Muslim]
As regards “when exactly the month of Ramadan will start, we cannot determine this now, but you have to fast with the Muslim community where you live based on the sighting of the crescent in your country or in the nearest Muslim country to your country where you live. It is normally broadcast through TVs and radios in all Muslim countries.
Allaah Knows best.