it starts from Fajr on the day of ‘Arafah and lasts until the sun sets on the last day of the days of Tashreeq, in addition to the takbeer that may be recited at any time. So when a person says the tasleem at the end of the obligatory prayer, he should pray for forgiveness (by saying Astaghfir-Allaah) three times, and say, “Allaahumma anta al-salaam wa minka al-salaam, tabaarakta yaa dhaa’l-jalaali wa’l-ikraam (“O Allaah, You are al-Salaam [the One Who is free from all defects and deficiencies), and from You is all peace, blessed are You, O possessor of majesty and honour)”, then start the takbeer.
This is for those who are not on Hajj. The pilgrims should start the takbeer recited after prayer from Zuhr on the Day of Sacrifice (the 10th of Dhu’l-Hijjah).
And Allaah knows best.