Samhain of Ancient Celts
During the days of the ancient Celtic people, [who occupied the area that is now Ireland, the United Kingdom and Northern France some 2,000 years ago], the year was dissected by the four seasons -- the last of which they prepared for on November 1st:
Large bonfires were raised in communities where celebrations included dancing in costumes consisting of animal heads and skins.  When the festivities concluded, each family would bring home a burning piece of the ceremonial fire. They would use the flames to reignite their hearths, blessing the fire in honor of the sun; hopeful that its warmth and light would get them through the cold, dark winter nights.
Lemuria of Pagan Rome
Ancients of pre-Christian Rome celebrated the festival of Lemuria on May 13th. During this "Lemuria/Lemuralia", rites were performed to exorcise the lemures [restless malevolent spirits]. Priestesses of the Goddess Vesta [goddess of the hearth] would prepare offerings of beans and sacred flour cakes [made from the first ears of seasonal wheat]. 
Conversion Intentions by Catholic Popes
So what link connects Halloween to a Roman festival in spring and Celtic celebration in fall? Enter: The Catholic Church.
Around 609-610 Pope Boniface IV dedicated the Pantheon in Rome to the Virgin Mary and all Martyrs, converting it into a Christian Church and thusly establishing the feast of dedicatio Sanctae Mariae ad Martyres. 
This is the first known instance in Rome of transformation of a Pagan temple into a site for Christian worship. The date of this conversion, May 13th, is not likely to be coincidental and is believed to have occurred in attempt by the Catholic Church to de-Paganize Rome.
Pope Gregory III added saints to the celebration of Boniface IV's martyrs, calling it All Saint's Day [or All Hallowmas from the Middle English for Alholowmesse]. 
Over time, Samhain celebrations merged with those enjoyed on the eve of All Saint's Day, called All Hallow's Eve and, eventually, 'Halloween' [first attested in the 16th century]. (  The Oxford English Dictionary (2nd ed.). Oxford: Oxford Univ. Press. 1989. ISBN 0-19-861186-2.)
We often hear from conservative Christians about the evil Pagan roots of our modern holiday and yet the irony proves hysterical when we consider that the day is celebrated today because of its connections to the Catholic Church and its intentions to convert the Romans, Celtics and Gaels by overshadowing local festivals with comparable observances of their own.
Speaking of conservative Christians...
Recently a very slanderous and ignorant opinion article was posted in Business Week unleashing harsh and inaccurate depictions of Paganism's grips on an otherwise Christian-West [through the celebration of Halloween]. 
Truth be told, Halloween and Samhain are two separate holidays, among many across the globe, which happen to fall on the same date during the year.
When will we see a time when supposed reputable sources for news, such as Business Week, demand more [ie, the truth] from authors submitting articles to their publications? Let's leave the spewing of lies to the tabloids, shall we?!
Happy Halloween, Samhain, Day of the Dead, Ten Chieh, Yue Lan, Alla Helgans Dag & many many more!