It is Sunnah to recite takbeer at any time during the first ten days of Dhu’l-Hijjah and all the days of Tashreeq, starting from the beginning of the month of Dhu’l-Hijjah (i.e., from sunset on the last day of Dhu’l-Qa’dah) until the end of the days of al-Tashreeq, which is when the sun sets on the thirteenth day of Dhu’l-Hijjah).
With regard to at certain times, 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 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.
[See Majmoo’ Fataawa Ibn Baaz (may Allaah have mercy on him), 13/17; al-Sharh al-Mumti’ by Ibn ‘Uthaymeen (may Allaah have mercy on him), 5/220-224.] no need to say..