Question: Does I’tikaf have a fixed time or is it restricted to Ramadan, or is it permissible in other than Ramadan?
A:What is prescribed is that it should be in Ramadan only.
This is because the Prophet ﷺ did not make I’tikaf in other than Ramadan, except what he ﷺ did in Shawwaal, after having left doing I’tikaf in Ramadan one year, and so did it in Shawwaal.
Yet if a person was to make I’tikaf in other than Ramadan it would be permissible, because ‘Umar (may Allah be pleased with him) asked the Prophet ﷺ saying: “I vowed to do I’tikaf for a night or a day in the mosque al–Haraam.” The Messenger of Allah ﷺ said: “Fulfill your vow”.
Sheikh Ibn ‘Uthaymeen, Fatawa Ramadan – Volume 2, Page 864, Fatwa No.851; Fiqh al–Ibaadaat libni ‘Uthaymeen – Page 208