It is recommended to recite Surat al-Kahf completely the night before Friday, and on Friday itself, before Maghrib time. There are many saheeh hadith which mentioned about the superiority of reciting Surah Al-Kahf on Friday:
Related by Hakim and Bayhaqi, from Abu Sa`id (Allah be pleased with him),
"Whoever recites Surat al-Kahf on Friday, light shall shine forth for him between the two Fridays."
[Ibn Hajar, Talkhis al-Habir]
Ibn 'Umar reports that the Prophet (saws) said:
"Whoever recites Surah al-Kahf on Jumu'ah will be blessed with a light that will rise from underneath his feet to the peak of the sky. This will be a light for him on the Day of Resurrection, and he will be forgiven for what is between the Jumu'ah [and the next] Jumu'ah."
Related by Ibn Mardwwiyah.
Al-Tirmidhi Hadith 2175 Narrated by Abu Sa'id: The Prophet (saws) said,
"If anyone recites Surah al-Kahf on Friday, a light will shine brightly for him till the next Friday."
Related by Bayhaqi.
From Abu Sa’eed al-Khudri, who said:
“Whoever readsSoorat 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)
“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. Ibn Hajar said inTakhreej al-Adhkaar that this is a hasan hadeeth, and he said, this is the strongest report that has been narrated concerning reading Soorat al-Kahf. See: Fayd al-Qadeer, 6/198. It was classed as saheeh by Shaykh al-Albaani in Saheeh al-Jaami’, 6470)
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-Mundhiri said, this was narrated by Abu Bakr ibn Mardawayh in his Tafseer, with an isnaad with which there was nothing wrong.
(al-Targheeb wa’l-Tarheeb, 1/298)
Sources:
http://qa.sunnipath.com/issue_view.asp?HD=1&ID=782
http://www.islamhelpline.com/node/4979
http://southernmuslimah.wordpress.com/2008/02/29/reminder-the-virtues-of-reciting-surah-al-kahf-on-friday/