It is the day on which the religion was perfected and Allah's Favour was completed.
"This day I have perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion" (Al-Maida 5:3)
It is on this day that the Prophet Muhammad delivered his final sermon to the largest gathering of companions. Never did he address such as gathering after it as he died shortly after.
Fasting on this day is an expiation for two years worth of minor sins.
Abu Qutaada reported that the Messenger of Allah was asked about fasting on the Day of Arafah. He said, "It expiated for the sins of the previous year and of the coming year." (Sahih Muslim).
It is the day of forgiveness of sins, freedom from the Fire and pride in the people who are there: Aisha reported that the Prophet Muhammad said, "There is no day on which Allah frees more people from the Fire than the Day of Arafah. (Sahih Muslim). He comes close and expresses His pride to the angels, saying, "What do these people want?" (Sahih Muslim). Use this day to ask for forgiveness and also make plenty of dua as there is no greater day to make doa that the day of Arafah.
This year the day of Arafah falls on Thursday 31st August 2017 which is today. Do not forget to fast on this day in order to get the expiation for two years' sins. May Allah accept it from us all. Amin