What is the validity of i'tikaf outside of Ramadan?

I'tikaf (spiritual retreat) is valid outside of Ramadan, due to the generality of Allah's saying: (And do not have sexual relations with them while you are in i'tikaf in the mosques), and His saying: (Purify My House for those who circumambulate it, and those who seclude themselves therein for devotion, and those who bow and prostrate themselves). The Prophet, peace and blessings be upon him, also performed i'tikaf in Shawwal, as narrated by Al-Bukhari.