Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- The first 10 days of Dhul-Hijjah are an opportunity for every Muslim to get abundant rewards.
3- If a Muslim intends to sacrifice an animal on the day of `Eid Al-Adha, then it is highly recommended neither to cut one’s nails nor to trim one’s hair.
In his response to your question, Dr. Zulfiqar Ali Shah, member of the Executive Committee of the Fiqh Council of North America and the Religious Director of the Islamic Society of Milwaukee, stated,
Trimming Hair in Dhul-Hijjah
Prophet Muhammad (peace and blessings be upon him) said, “If one of you intends to sacrifice an animal on the day of `Eid al-Adha, then he or she should stop cutting the hair and the nails.” (Muslim)
Imam Malik, Imam Ahmad, and Imam Shafi`i (in one of his opinions) strictly follow this hadith and forbid cutting the nail or hair during the first 10 days of the month of Dhul-Hijjah.
The Hanafi School declares it as Sunnah mu’akkadah (Arabic for: confirmed sunnah) and highly recommends that a Muslim holds off cutting the nail and hair and to do so after sacrificing the animal, so that the hair and the nails are counted as hasanat (Arabic for: good deeds) along with the hair of the sacrificial animal.
It is not haram to cut the hair or the nails; but it is recommended so that one can achieve additional rewards for good deeds.
Allah Almighty knows best.