The white days of the month refer to the 13th, 14th and 15th of the Islamic month, when the moon is at its fullest and most luminous. “Beedh” is the plural of “Abyad” which means white. The 13th, 14th and 15th of the lunar month are known as “Ayyamul Beedh”. These nights are brighter and the moon begins to diminish gradually after these days.
Fasting on these 3 days is an established Sunnah, mentioned in the Ahadith:
Nabi (Sallallahu Alaihi Wasallam) said, “Shall I not tell you of something that will take away the evil of the heart? Fasting three days of each month.” (Sunan an-Nasa’i)
Hazrat Abdullah bin Abbas (R) narrates, “Nabi (Sallallahu Alaihi Wasallam) never used to miss the fast of the Ayyamul Beedh (white days) whether at home or on journey.” (Sunan an-Nasa’i)
Hazrat Jareer bin Abdillah (R) narrates that Nabi (Sallallahu Alaihi Wasallam) said, “Fasting three days of each month is fasting for a lifetime, (the days being) the shining days of whiteness, the 13th, 14th and 15th.” (Sunan an-Nasa’i)