Praise be to Allaah.
There are saheeh ahaadeeth from the Prophet (peace and blessings of Allaah be upon him) concerning the virtues of reciting Soorat al-Kahf during the day or night of Jumu’ah (Friday). These include:
(a) From Abu Sa’eed al-Khudri, who said: “Whoever reads Soorat al-Kahf on the night of Jumu’ah, will have a light that will stretch between him and the Ancient House (the Ka’bah).”
(Narrated by al-Daarimi, 3407)
(b) “Whoever reads Soorat al-Kahf on the day of Jumu’ah, will have a light that will shine from him from one Friday to the next.”
(Narrated by al-Haakim, 2/399; al-Bayhaqi, 3/249)
(c) It was narrated that Ibn ‘Umar (may Allaah be pleased with him) said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Whoever reads Soorat al-Kahf on the day of Jumu’ah, 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-Targheeb wa’l-Tarheeb, 1/298)
The soorah 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 soorah extends from sunset on Thursday to sunset on Friday.