Not celebrating, exactly. I guess you could call it the Muslim equivalent of Lent. It is a month of every year where muslims must fast, and by fasting I mean that they are not allowed to eat or drink from dawn till dusk (heh). Same goes for sex. They're also supposed to be at their most pious, no profanity, gossip, or any of the little daily sins. They should give to the poor, spend as much time as possible praying and reading Quran. It's supposed to teach self control and discipline, as well as remind us how the poor fare every day of their lives.