Arafah (9th Zul-Hijjah).
Regarding this day Rasulullah (Sallallahu Alayhi Wasallam) is reported to have said: “I have hope in Allah Ta’ala that the fast of the day of Arafah will be an atonement for the (minor) sins of the past and forthcoming year” (Tirmizi).
(full Hadith)