It is permissible for a Muslim to perform ‘Umrah during the months of Hajj as there is no difference of opinion amongst the scholars concerning that. It makes no difference whether one intends to do Hajj in the same year or not.
The Prophet (peace and blessings be upon him) did ‘Umrah four times and all of them were in the month of Dhu’l-Qi’dah, which is one of the months of Hajj namely; Shawwal, Dhu’l-Qi’dah and Dhu’l-Hijjah. He did not perform Hajj except along with his last ‘Umrah, in his Farewell Pilgrimage.
