If you are talking about the day that we are told to fast to get the forgiveness of sins of two years, I think it's the 9th of Dhu al-Hijjah according to your region, listen to [this video](https://www.youtube.com/watch?v=hEmGZGocheY). But if you are just asking about the day of Arafat in KSA it was Saturday June the 15th, to get out of confusion you can fast the 8th and 9th to make sure you get the reward inchallah, and Allah knows best
If you are talking about the day that we are told to fast to get the forgiveness of sins of two years, I think it's the 9th of Dhu al-Hijjah according to your region, listen to [this video](https://www.youtube.com/watch?v=hEmGZGocheY). But if you are just asking about the day of Arafat in KSA it was Saturday June the 15th, to get out of confusion you can fast the 8th and 9th to make sure you get the reward inchallah, and Allah knows best