In the name of Allah, Most Merciful, Bestower of Mercy.
We hope that there is no sin upon you because your intention is not to single out Friday with fasting. The purpose of your fast is only because it is the Day of ʿArafah. However, if you fast the day before it also (i.e. Thursday) then that is safer because the Prophet (salallāhu ʿalaihi wasallam) prohibited us from singling out Friday with optional fasting. So if you were to fast Thursday alongside Friday, then that is safer even if you intend the Day of ʿArafah and not Friday itself.
As for singling out Friday with fasting due to the virtue of Friday, then that is not permitted because the Prophet (salallāhu ʿalaihi wasallam) forbade that. However, if he fasts it on its own because it is the Day of ʿArafah, then we hope that there is no issue with that, but being prudent by fasting Thursday along with it is safer.
(Source Nūr ʿalad-Darb slightly abridged)
Ibn Qudāmah (rahimahullāh) said in al-Mughnī: “A man fasts one day and withholds the next day (and carries on in that manner). So his withholding coincides with a Thursday and his fasting coincides with a Friday, and then his withholding coincides with Saturday. So does he fast Friday on its own? This person did not deliberately single out Friday for his fast. It is only hated if he deliberately intends Friday on its own.“
In summary, it is allowed to fast on a Friday on its own if ’Arafah coincides with it because the person has not deliberately singled out Friday.
Fasting on a Saturday if ‘Āshūrā or ‘Arafah happens to fall on it: Ibn ʿUthaimeen
Ibn Uthaimeen (rahimahullāh) on Fasting on a Saturday
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