The three brightest nights of the Islamic month, the 13th, 14th and 15th, are called the white days, and they are traditional fasting days.

Ayyām al-bīḍ are named for the moonlight, which by that point in the cycle keeps the ground pale most of the night. The fast is recommended rather than required and recurs monthly with the phase. It is one of the clearest examples anywhere of a religious calendar organised around how much light there is.