Why do we fast the six white days?

Fasting six days of Shawwal is commendable and is equivalent to fasting a whole year for whoever fasts Ramadan and then follows it with six days from Shawwal, because of the Prophet's (peace be upon him) saying: "Whoever fasts Ramadan and then follows it with six days from Shawwal, it is as if he has fasted for a lifetime."

As for fasting the White Days of each month—which are the thirteenth, fourteenth, and fifteenth—this also is equivalent to fasting for a lifetime.