Why are 13th 14th and 15th of the lunar month known as Ayyamul Bid?
Literally this translates as: The days of brightness.
‘Allamah ‘Ayni (and others rahimahumullah) explain that this term is given due to the 13th to 15th nights being the brightest in the entire month. This is because the moon’s complete size is visible throughout the night.
Therefore, this should be understood as: Ayyamu layalil bid, days of the bright nights.
(‘Umdatul Qari, before Hadith: 1981 and Tuhfatul Bari, before Hadith: 1981)
Hafiz Ibn Hajar (rahimahullah) has stated that some have offered other explanations, but those are not as well substantiated.
(Fathul Bari, before Hadith: 1981).
See here for the Hadith on fasting on these days.
And Allah Ta’ala Knows best,
Answered by: Moulana Muhammad Abasoomar