Fasting on the Day of আরাফাত (the 9th of Dhul Hijjah) is one of the most virtuous voluntary fasts in Islam for those who are not performing হজ্জ. নবী করীম (সাল্লাল্লাহু আলাইহি ওয়া সাল্লাম) was asked about fasting on the Day of আরাফাত and said: 'It expiates the sins of the previous year and the coming year.' This tremendous reward makes it one of the most highly মুস্তাহাব acts of worship for non-হাজীগণ.
However, হাজীগণ who are actually at আরাফাত performing হজ্জ are advised NOT to fast on this day. নবী করীম (সাল্লাল্লাহু আলাইহি ওয়া সাল্লাম) himself did not fast on the Day of আরাফাত during his হজ্জ. The reason is practical and spiritual — the Day of আরাফাত requires immense physical and spiritual energy. Pilgrims are standing in extreme heat for hours, making intense দোয়াs, and need their strength. Fasting could compromise their ability to worship fully. There was even a narration where Umm al-Fadl sent a container of milk to নবী করীম at আরাফাত, and he drank it publicly to show people he was not fasting.
For non-হাজীগণ around the world, this fast is separate from the Days of Tashreeq (11th-13th Dhul Hijjah), on which fasting is prohibited. The Day of আরাফাত fast is standalone and carries its immense reward independently. Combined with increased dhikr, dua, and charity on this day, non-হাজীগণ can spiritually participate in the blessings of the হজ্জ season from wherever they are.