The Prophet ﷺ said: 'The best dua is the dua of the Day of Arafah.' It is the day on which Allah frees more people from the Fire than on any other.
Pilgrims stand at Arafah from after Zuhr until sunset, facing the qiblah, hands raised, in continuous dua and dhikr.
Standing at Arafah is the pillar without which there is no Hajj: 'Hajj is Arafah.'
Fast the Day of Arafah — it expiates the sins of the previous year and the coming year.
Spend the afternoon in dua until Maghrib, and increase in the takbir of Dhul Hijjah.
لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
La ilaha illa Allah wahdahu la sharika lah, lahu al-mulku wa lahu al-hamdu wa huwa 'ala kulli shay'in qadir
There is no deity but Allah alone, with no partner. To Him belongs the dominion and all praise, and He is capable of all things.
Sunan al-Tirmidhi 3585
رَبَّنَا آتِنَا فِي الدُّنْيَا حَسَنَةً وَفِي الآخِرَةِ حَسَنَةً وَقِنَا عَذَابَ النَّارِ
Rabbana atina fid-dunya hasanah wa fil-akhirati hasanah wa qina 'adhab al-nar
Our Lord, give us good in this world and good in the Hereafter, and protect us from the punishment of the Fire.
Qur'an 2:201
Scholars differ. Many advise following your local Dhul Hijjah calendar, while others follow the day of standing at Arafah. Either position is defensible.
The most emphasised time is from after Zuhr until sunset, with the final hour before Maghrib being especially precious.