Reference: Fataawa Ramadhaan – Volume 2, Page 872, Fatwa No.859
Fataawa ash-Shaykh Muhammad as-Saalih al-‘Uthaymeen – Volume1, Page 551
Question: When does a person leave i’tikaaf, is it after sunset on the night prior to ‘Eed or after Fajr on the day of ‘Eed?
Response: The person in i’tikaaf leaves i’tikaaf once Ramadhaan finishes and it finishes as the sun sets the night prior to ‘Eed. He enters i’tikaaf at sunset [prior] to the 20th day of Ramadhaan. This is so as the last ten [nights] of Ramadhaan start from sunset of the night [prior] to the 20th day of Ramadhaan and ends at sunset of the night [prior] to ‘Eed.