What are the reasons behind naming the 13th, 14th, and 15th days of the Arabic month as “the white days” (Al-Ayyam Al-Beed), what is the difference between Dhuhr prayer and Jumu’ah prayer, and what is the meaning of washing the knuckles (ghusl al-barajim) and istinjaa’?

The reason for naming the thirteenth, fourteenth, and fifteenth days of the lunar month as "Ayyam al-Bid" (the White Days) is the completion of the moon and its clear appearance throughout these nights, from their beginning to their end, making them white and luminous. For this reason, they were called "Ayyam al-Bid," in addition to "Bid" (white) being an attribute of the nights.