بسم الله الرحمن الرحيم
Let us glorify Allaah with Takbeer
From the day of ’Arafah (the 9th of Dhul-Hijjah), up until the ’Asr Prayer on the thirteen day, are days in which the takbeeraat (saying Allaahu Akbar) should be said. Imaam al-Khattaabee (d.456H) – rahimahullaah – said, “The wisdom behind saying the takbeeraat in these days is that in the times of jaahiliyyah (pre-lslaamic ignorance), they used to slaughter for their tawaagheet (false objects of worship). So the takbeeraat were prescribed in order to indicate that the act of slaughtering is directed to Allaah alone, and by mentioning only His – the Mighty and Majestic – Name.”
Quoted from Fathul-Baaree (21/586).