Some special forms of worship for these 10 days
In these blessed days Hajj is performed and those who are performing Hajj are in a state of Ihraam. Thus, to develop some kind of resemblance with them, for those who are performing sacrifice of an animal, it is commendable not to cut their hair or trim their nails. The Holy Prophet (Sallallahu alaihi wa sallam) has prohibited us from doing so. There is a Hadith in Sahih Muslim:
It is narrated on the authority of Hadhrat ‘Umar bin Muslim bin ‘Ammaar (May Allah be well pleased with him), he says: I heard Hadhrat Sa‘eed bin Musayyib (May Allah be well pleased with him) say: I heard the mother of the believers Hadhrat Umm Salma (May Allah be well pleased with her) say that the Holy Prophet (Sallallahu alaihi wa sallam) said: The person who has an animal for sacrifice, on seeing the moon of Dhul Hijjah, should neither remove/cut hair nor cut nails until he slaughters it.
(Sahih Muslim, Hadith No. 5236)