Is it permissible to perform Umrah at any time of the year, or is it restricted to the months of Hajj?

It is permissible to perform 'Umrah on all days of the year, even during the months of Hajj. So, whoever performs 'Umrah in the months of Hajj and then performs Hajj in the same year is a Mutamatti', and an offering (hady) is obligatory upon him unless he is among those residing in the vicinity of the Sacred Mosque. And whoever performs 'Umrah along with his Hajj is a Qaarin, and an offering (hady) is also obligatory upon him unless he is among those residing in the vicinity of the Sacred Mosque. However, whoever performs 'Umrah in Dhul-Hijjah after the Days of Tashreeq, no offering (hady) is obligatory upon him.