Reference: Fataawa Ramadhaan – Volume 2, Page 864, Fatwa No.851
Fiqh al-Ibaadaat libni ‘Uthaymeen – Page 208
Question: Does i’tikaaf have a fixed time or is it restricted to Ramadhaan, or is it permissible in other than Ramadhaan?
Response: What is prescribed is that it should be in Ramadhaan only. This is because the Prophet (sal-Allaahu ‘alayhi wa sallam) did not make i’tikaaf in other than Ramadhaan, except what he (sal-Allaahu ‘alayhi wa sallam) did in Shawwaal, after having left doing i’tikaaf in Ramadhaan one year, and so did it in Shawwaal. Yet if a person was to make i’tikaaf in other than Ramadhaan it would be permissible, because ‘Umar (radhi-yAllaahu ‘anhu) asked the Prophet (sal-Allaahu ‘alayhi wa sallam) saying: “I vowed to do i’tikaaf for a night or a day in the Masjid al-Haraam.” The Messenger of Allaah (sal-Allaahu ‘alayhi wa sallam) said:
«Fulfil your vow».