"Fasting three days of each month is fasting for a lifetime, and the shining days of Al-Bid, the thirteenth, fourteenth and fifteenth."
Hadith Commentary: The Virtue of Monthly Fasting
This narration from Sunan an-Nasa'i 2420 establishes the immense reward for fasting three days monthly, equating it to perpetual fasting in divine recompense, while specifying the optimal days as the 13th, 14th, and 15th of the lunar month, known as Ayyām al-Bīḍ (The White Days).
Scholarly Explanation of "Fasting for a Lifetime"
Scholars explain that this tremendous reward stems from the divine principle that one good deed is multiplied tenfold. Thus, three days monthly equal thirty days in reward, effectively covering an entire year's worth of fasting in spiritual merit, though not replacing the obligatory fast of Ramadan.
Wisdom Behind Ayyām al-Bīḍ (The White Days)
These middle days of the lunar month are called "white" because the moon is fully illuminated during the nights. Classical commentators note this timing coincides with peak lunar brightness, symbolizing spiritual illumination. Furthermore, these days correspond with historical divine acts, including the salvation of Prophet Yunus and acceptance of Prophet Adam's repentance.
Practical Juridical Rulings
The majority of scholars consider this fasting highly recommended (sunnah mu'akkadah). While these specific days are optimal, the general reward applies to any three monthly days. If missed, one may fast any other three days. This practice maintains continuous spiritual connection throughout the year without undue hardship.