The strongest opinion on why they are called Ayyam-ut-tashreeq or the days of Tashreeq is because prior to the invention of the refrigeration system, the pilgrims used to slice the meat they acquired from the udhiyah, season it with salt and then let it dry in the sun…
I have the habit of fasting every Monday and Thursday. I heard that it is not permissible to fast on the Days of Tashreeq (11, 12, and 13 Dhul-Hijjah). I fasted one of the Days of Tashreeq.
I understand that it is an act of Sunnah to fast on the White Days of every month (i.e., the 13th, 14th, and 15th of each lunar month).