The Messenger of Allah ﷺ said, "For Fasting the Day of 'Arafa, I anticipate that Allah will expiate the [sins] of the previous and upcoming year." [Muslim]
Righteous scholars of the Umma also recommend some of the following Invocations:
The Prophet ﷺ said: “The best prayer is the prayer of the Day of ‘Arafa. The best thing which I and the Prophets before me have said is:
لاَ إِلَهَ إِلاَّ اللهُ وَحْدَهُ لاَ شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَ لَهُ الْحَمْدُ وَ هَو عَلَى كُلِّ شَيءٍ قَدِيرٌ
La ilaha ill’Allahu wahdahu la sharika lahu, lahu’l-mulku wa lahu’l-hamdu wa `ala kulli shayin qadir [“There is no deity save Allah alone. He has no partners. To Him belongs the dominion and all praise and He has power over all things.”]
It is also recommended to recite Sura al-Ikhlas (Qul Huwa Allahu Ahad) 1000 times and to make abundant duʿa on this most blessed day.
(Please note that the exact date will be determined based on moon sighting)