The word Samhain is derived from Old Irish and refers to the first of November. Samhain is the Festival of the Dead and symbolizes the end of the summer and the Celtic New Year. Traditionally, it was the day of the last harvest and the time to take inventory of live stock to be slaughtered.
According to Celtic lore Samhain is the time of year when the boundaries between the worlds of the living and the dead become transparent, allowing supernatural beings to pass between the worlds. Usually, it's the time of year where ancestors were especially honored (see "All Saints' Day). The Samhain Festival involved bonfires, as well as costumes and masks to disguise the living and make them mimic the evil spirits that walked the earth on Samhain.
Many Halloween traditions come from Irish and Scottish Lore - the carving of the pumpkins and lighting candles inside of them, the disguising as supernatural or evil creatures, the fireworks and bonfires. Trick-or-Treating derives from the tradition of Mischief Night. At Hollywood parties, typical games are dunking for apples or eating strung up scones without the use of hands.