Narrated Abi Sa'id al-Khidri from Prophet Muhammad (Peace Be upon Him) that He 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. This hadeeth was classed as saheeh by Shaykh al-Albaani in Saheeh al-Jaami, 6471)
In another narration: “Whoever reads Soorat al-Kahf on the day of Jumu’ah, will have a light that will shine for him from one Friday to the next.” (Narrated by al-Haakim, 2/399; al-Bayhaqi, 3/249. It was classed as saheeh by Shaykh al-Albaani in Saheeh al-Jaami’, 6470).
Al-Tayibi said: Daylight shining between two Fridays is like the shining of light itself. (Fayd al-Qadeer, 6/198).
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.