Is Iʿtikāf observed at any time or only during Ramadan?

I'tikaf (spiritual retreat) is a Sunnah at all times, and Ramadan is superior for it due to the Ahadith (prophetic narrations) reported in this regard. The most emphasized time for it is the last ten nights of Ramadan. The Prophet (peace be upon him) observed I'tikaf for ten days in Shawwal, and Umar (may Allah be pleased with him) vowed to observe I'tikaf for one night in the Sacred Mosque. I'tikaf includes both Ramadan and other times.