There are confirmed aḥādīth when it comes to the virtues of sūrat al-Kahf, but when it comes to the ḥadīth which states:
مَنْ قَرَأَ سُورَةَ الْكَهْفِ يَوْمَ الْجُمُعَةِ، أَضَاءَ لَهُ مِنَ النُّورِ مَا بَيْنَهُ وَبَيْنَ الْبَيْتِ الْعَتِيقِ
Whoever recites sūrat al-Kahf on the day of Jumuˋah will have illumination from light in what is between himself and al-Bayt al-ˋAtīq – [the Ancient House, meaning the Kaˋbah]
(or in similar wordings) then we should know the following:
• The narrations can only be confirmed by way of the Ṣaḥābī Abū Saˋīd al-Khudrī (رضي الله عنه). There is nothing authentic from any of the other Ṣaḥābah.
• The only isnād (chain) to the narration of Abū Saˋīd al-Khudrī (رضي الله عنه) comes by way of Abū Hāshim, from Abū Mijlaz, from Qays ibn ˋAbbād, from Abū Saˋīd al-Khudrī (رضي الله عنه). Those who narrated it from Abū Hāshim then differed concerning it.
• Those who narrated the ḥadīth in marfūˋ form – meaning being the words of the Prophet (ﷺ) directly – then these narrations are defective or incorrect. The confirmed narrations are in mawqūf form being from the words of Abū Saˋīd al-Khudrī (رضي الله عنه) and not the words of the Prophet (ﷺ).
• As for whether the mawqūf narration of Abū Saˋīd al-Khudrī can have the ruling of being marfūˋ – meaning he would not have said this unless he heard this from the Prophet (ﷺ), then there are elements of knowledge in the ḥadīth pertaining to future events that a Ṣaḥābī would not say of his own accord (such as the seal being placed under the Throne, the seal not breaking until the Day of Resurrection, protection from the Dajjāl, and the details of the light).
• The addition of “on the day of Jumuˋah (Friday)” in the narrations is munkar – a rejected addition. This can only be found in the confirmed narrations from Hushaym ibn Bashīr from Abū Hāshim. Both the narrations of Shuˋbah ibn al-Ḥajjāj and Sufyān ath-Thawrī from Abū Hāṣhim do not include this addition. Although Hushaym is thiqah (reliable), Yaḥyā ibn Saˋīd al-Qaṭṭān said, “Hushaym is the most-established in memorization that I have seen after Sufyān and Shuˋbah.” There is also doubt concerning whether or not Hushaym heard this narration from Abū Hāshim.
• The addition of “on the night of Jumuˋah” is even more rejected than “on the day of Jumuˋah.”
• What is confirmed from the narrations is that the light will be from where the person stands to Makkah. As for the light being from one Friday to the next, or between the person and the heavens, or even that the recitation of it will expiate what is between the two Fridays, then these are all rejected.
• The virtue of the light is from the recitation of the entire sūrah, as most of the narrations agree, and not just from reciting the end of it as is found in one narration from ˋAbd-ur-Razzāq. It would appear he made a mistake – perhaps confusing this with the narrations having to do with the protection from the Dajjāl.
• Since the virtue is not restricted to a time or place, one can hope to attain it anytime, and is encouraged to recite sūrat al-Kahf whenever possible and as much as possible.
And Allāh knows best.