“The virtues of fasting are immense, and a number of authentic hadiths state this, such as the following:
The Prophet (peace and blessings be upon him) is reported to have said: “Whoever fasts one day for Allah’s sake, Allah will keep his face seventy years’ distance away from the Fire because of it.” (Reported by Muslim)
The Prophet (peace and blessings be upon him) is also reported to have said: “Whoever fasts a day seeking the pleasure of Allah and then dies (i.e., having the fasting as the last thing he does) will enter Paradise.” (Reported by Ahmad).
The White Days do exist in every lunar month (namely, the 13th, 14th, and 15th of each Hijri month). Muslims are highly recommended to fast on these three days each month. Ibn `Abbas said, "The Prophet (peace and blessings be upon him) would habitually fast on the White Days, whether he was resident or on a journey."
Hafsah is also reported to have said, "There are four things the Prophet (peace and blessings be upon him) never abandoned: fasting on the day of `Ashura’, on the first 10 days of Dhul-Hijjah, and on three days of every month, as well as [performing] the two rak`ahs of duha [forenoon]" (reported by Ahmad).
"None of you should fast on Friday, unless he [or she] fasts on either one day before it or one day after it" (Al-Bukhari and Muslim).
The Muslim scholars agree that fasting on Saturday alone is disliked in light of the following hadith of the Prophet: "Do not fast on Saturday, unless it [such fasting] is part of what Allah has prescribed for you. And if you could not find anything to eat but grape skin or a piece of wood, you should chew it." (At-Tirmidhi)
As for fasting Mondays and Thursdays, Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (peace and blessings be upon him) used to fast on Monday and Thursday. On being asked about that the Prophet said: "Deeds are presented on every Monday and Thursday. Allah forgives every Muslim or every believer, except for those who are forsaking each other. He says [about them]: 'Leave them.' " (Reported by Ahmad with authentic chain of narrators)
It is recorded in Sahih Muslim that the Prophet (peace and blessings be upon him) when asked about fasting on Monday, said: "That is the day on which I was born and the day on which I received revelation."
For those desiring the maximum fasting experience, the prophet(peace be upon him) recommended the fast of Prophet David, every other day. He did not permit continual daily fasting in order that fasting does not become routine and automatic. Fasting every other day keeps the freshness of the fast alive.