Ayyām al-Bīḍ — The White Days (Ayyām al-Bīḍ)

The White Days (Ayyām al-Bīḍ) — the 13th, 14th and 15th of the Hijri month — are the most virtuous days to keep a voluntary fast; they are named after the bright nights of the full moon.

Qatādah ibn Milḥān al-Qaysī reported: "The Messenger of Allah ﷺ used to command us to fast the white days — the thirteenth, fourteenth and fifteenth of the month. He said: This is like keeping perpetual fast." (Sunan Abī Dāwūd 2449)

ʿAbdullāh ibn ʿAmr ibn al-ʿĀṣ reported that the Prophet ﷺ said: "It is enough for you to fast three days every month, for every good deed you have ten like it, so that is like fasting for a lifetime." (Ṣaḥīḥ al-Bukhārī 1975)

Note: the 13th of Dhū al-Ḥijjah is a day of Tashrīq, on which fasting is not permitted; the White Days here are the 14th and 15th.