Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.122B.

The majority of scholars is of the opinion that ‘Umrah may be performed any time during the year. Abu Hanifah, however, is of the opinion that it is disliked to perform ‘Umrah on five days: the Day of ‘Arafah, the Day of Nahr (the 10th of Zhul-Hijjah), and the three days of Tashriq (11th, 12th, and 13th of Zhul-Hijjah).

Abu Yusuf considers it disliked on the Day of ‘Arafah, and during the three following days. All of them, however, agree on the permissibility of performing ‘Umrah during the months of Hajj.

Bukhari has reported from ‘Ikrimah bin Khalid that he said, “I asked Abdullah bin ‘Umar about performing ‘Umrah before Hajj, whereupon he said, ‘There is no harm in doing so, because the Prophet (peace be upon him) himself performed ‘Umrah before the Hajj.”‘

Jabir reported that ‘Aishah’s menstruation started while she was performing Hajj, but she performed all the rites of Hajj except tawaf around Ka’bah. When her period ended and she was clean, she performed the tawaf, and said, “O Prophet of Allah! Everyone has performed Hajj and ‘Umrah together, but I have performed Hajj only.” Thereupon the Prophet (peace be upon him) commanded her brother Abdur Rahman to accompany her to Tan’im (in order to perform ‘Umrah). So she performed ‘Umrah right after Hajj in the month of Zhul-Hijjah. The best time to perform ‘Umrah is during the month of Ramadan.