The hajj takes place during Dhu'l-Hijja, the last month of the Muslim calendar year (which is based on lunar cycles, meaning the hajj dates change annually).    From Wordnik.com.  [10 Gorgeous Pilgrimage Sites You Need to See] Reference

"When any one of you intending to sacrifice the animal enters in the month (of Dhu'l-Hijja) he should not get his hair or nails touched (cut)".    From Wordnik.com.  [Bloggers.Pakistan] Reference