What are the Days of Tashreeq, and what are their virtues?

The Days of Tashreeq are the eleventh, twelfth, and thirteenth days of Dhul-Hijjah. Their virtue has been mentioned in the Qur'an and Sunnah. Allah's saying: (And remember Allah during [specific] numbered days) refers to the Days of Tashreeq. The Prophet, peace and blessings be upon him, said about them: "Indeed, they are days of eating, drinking, and remembrance of Allah, the Mighty and Majestic." During these days, one should remember Allah by performing the Takbir after the prayers, and by saying "Bismillah" and "Allahu Akbar" when slaughtering the sacrifice, and by remembering Him while eating and drinking, and by saying the Takbir when stoning the Jamarat for pilgrims, and by frequently engaging in general remembrance (dhikr mutlaq) and supplication (du'a). The Prophet, peace and blessings be upon him, forbade fasting on these days.