Abu-Darda’ (radiyallahu anhu) reported: The Messenger of Allah (salallahu alayhi wasallam) said: “Whoever commits to memory the first ten Ayat of the Surat Al-Kahf, will be protected from (the trial of) Ad-Dajjal (Antichrist).” [Muslim]
In another narration, the Messenger of Allah (salallahu alayhi wasallam) said: “(Whoever commits to memory) the last ten Ayat of Surat Al-Kahf, he will be protected from (the trial of) Ad-Dajjal (Antichrist).”
Ad-Dajjal will appear sometime before Yaum-ul-Qiyamah. Allah, the Almighty will give him some supernatural powers the manifestation of which will shake the faith of many people. For this reason, this will be a serious trial. All Prophets (alayihimus-sallam) warned their people against him. Our Prophet (salallahu alayhi wasallam) also warned us against him and ordered us to guard ourselves from it and strictly advised us to seek Refuge of Allah against it.
It was narrated that Ibn ‘Umar (radiyallahu anhuma) said: “The Messenger of Allah (salallahu alayhi wasallam) said: ‘Whoever reads Surat al-Kahf on the day of Jumu’ah (Friday), a light will shine for him from beneath his feet to the clouds of the sky, which will shine for him on the Day of Resurrection, and he will be forgiven (his sins) between the two Fridays.’”
[Al-Mundhiri said, this was narrated by Abu Bakr ibn Mardawayh in his Tafseer, with an isnaad with which there was nothing wrong]
From Abu Sa’eed al-Khudri (radiyallahu anh), who said: “Whoever reads Surat al-Kahf on the night of Jumu’ah (Friday), will have a light that will stretch between him and the Ancient House (the Ka’bah).” [Narrated by al-Daarimi]
“Whoever reads Surat al-Kahf on the day of Jumu’ah (Friday), will have a light that will shine from him from one Friday to the next.” [Narrated by al-Haakim al-Bayhaqi].
The surah may be read during the night or the day of Jumu’ah. The night of Jumu’ah starts from sunset on Thursday, and the day of Jumu’ah ends at sunset. Therefore the time for reading this surah extends from sunset on Thursday to sunset on Friday.
It is recommended to recite Surat al-Kahf completely the night before Friday, and it is also recommended to do so Friday itself, before Maghrib time. Ibn Abidin said, And it is best to do so early on Friday, in order to avoid forgetting. [Ibn Abidin, Radd al-Muhtar, Bab al-Jumu`ah]