It is permissible to do ‘Umrah at any time of the year

It is permissible to do ‘umrah on all days of the year, even
during the months of Hajj. If a person does it during the months of Hajj and
does Hajj after that in the same year, then he is said to be doing tamattu’
for ‘Umrah and Hajj. If he does it with Hajj, then he is doing qiraan and
combining Hajj and ‘umrah. Both those who are doing tamattu’ and those who
are doing qiraan must offer a hadiy of an animal that is acceptable as an
udhiyah, if he is not a resident of Makkah. If the pilgrim does ‘umrah in
Dhu’l-Hijjah after the days of al-Tashreeq, it is permissible and he does
not have to offer a hadiy. 

And Allah is the source of strength. May Allaah send
blessings and peace upon our Prophet Muhammad and his family and companions.
End quote. 

Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, Shaykh ‘Abd al-Razzaaq al-‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan