Is Hallowe'en A Celtic Festival?

Yes, it is generally thought that Hallowe'en has its roots within the Celtic Culture.  For Celts, the New Year began on 1st November heralding the coming of winter, darkness and increased risk of death.  On the eve before this, the 31st October they held the festival of Samhain, the Celtic Lord of death.  Not to welcome him, but rather to keep him away!
They believed that on October 31st, he allowed ghosts to rise from their graves along with goblins, demons and other nasties.  Priests believed that burning a fire would keep the evil spirits away.  Food and animals were put on the fire to keep Samhain happy.  
People also dressed up in costumes made from animal skins and animal heads because they believed that this provided further protection against the evil spirits.  Hence the start of the 'dressing up' aspects of Hallowe'en and why some Church leaders are not in favour of this festival !