The one who intends to do a sacrifice should not cut his hair and nails from the beginning of Dhul-Hijjah UNTIL the sacrifice is done.
Shaykh Al-Albaanee, may Allah shower His Mercy upon him, states in our of his works:
The Messenger of Allah, sallallahu alayhi wa sallam said:
‘When the new moon of Dhul-Hijjah appears and one of you wants to do the sacrifice, then he should abstain from (cutting) his hair and his nails.’
And in a narration:
‘Then he should not take anything from his hair or from his nails until he sacrifices.’
– Mukhtasar Saheeh Muslim No.1251 and other than it.
Shaykh Al-Albaani, may Allah shower his Mercy upon Him, comments:
“I say: And the apparent meaning of the hadeeth is the obligation to leave removing the hair and the nails for the one who has resolved to do the sacrifice until he sacrifices … and this is what Imaam Ahmad and other than him have said.
So, let those who are afflicted with the shaving of the beard pay attention to this, for indeed shaving it (the beard) for ‘Eed has in it three acts of disobedience:
The First: The shaving in of itself, for indeed it is to become feminine and to resemble the disbelievers and to change the creation of Allah as I have explained it in my book Adaab-uz-Zifaaf Fis-Sunnatil-Mutahharah (6th Edition, page 118).
The Second: Beautifying (oneself) for the ‘Eed through the disobedience of Allah!
The Third: What this hadeeth has benefitted with from the forbidding of taking (from) the hair for the one who wants to sacrifice. And in reality, very few are those who are saved from these violations (of the religion), even some of the people of knowledge! We ask Allah for safety.”