The Day of Arafah
The Day of Arafah is held on May 15. It is an Islamic holiday that falls on the 9th day of Dhu al-Hijjah of the lunar. This event in the second decade of the month May is annual. 
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The Day of Arafah is known as the day where if sought after, millions of Muslims' sins are forgiven by Allah. The Day of ’Arafah is the ninth day of Dhul-Hijjah . Arafah is known as the day Allah perfected the religion of Islam. It is also momentous because it was the day an important verse of the Qur'an, Surah al Maa'idah 5:3, was revealed.
The first ten days of Dhul-Hijjah are the best days of the entire Islamic year. The Prophet (saw) said, ‘There are no days greater and more beloved to Allah than these (first) ten days of Dhul-Hijjah’.
Abu Qatada al-Ansari narrated that the prophet Muhammad was asked about fasting on the Day of Arafah, whereupon he said: It expiates the sins of the preceding year and the coming year.
Similar holidays and events, festivals and interesting facts
Remembrance Day for Martyrs and Disabled in Afghanistan on May 4
Kurban Hait or Eid-al-Adha or Kurban-bayram on May 16 (Uzbekistan, Afghanistan, Hari Raya Aidiladha in Brunei);
Muslim New Year on June 5 (The Islamic New Year, also known as Arabic New Year, Muharram or Hijri New Year is the day that marks the beginning of a new Hijri year);
Ashura on June 14 (is a major festival for Shia Muslims and commemorates the martyrdom at Karbala of Hussein, a grandson of the Prophet);
Engineer's Day in Bahrain on July 1
The Prophet's Birthday or Mawleed al Nabi on August 14 (or Mawlid al-Nabi. There is not a single verse in the Qur'an OR a Hadith of Prophet Muhammad (Peace be upon him) forbidding the celebration of Prophet's birthday);
Afghan Independence Day on August 19 (also Jeshyn-Afghan Day. Commemorates the Treaty of Rawalpindi in 1919, granting independence from Britain)