Samhain: (see above for diverging pronounciation)
Scottish Gaelic: Sam + Fhuin = end of summer (sam: summer; fuin: end)
It is important to point out that there is no Irish "god of the dead," and especially no god named "Samhain," as some rather erroneous books/websites will tell you. There is some argument as to whether one can consider Arawn or Gwyn ap Nudd Welsh gods of the dead, or at least psychopomps, but the Irish pantheon has no official god of the dead. (Some, though, hypothesize that Donn or Mil is this god, as Caesar says that the Gauls believed they were decended from Dis, the Roman name for the god of the dead, and sometimes Donn or Mil are named the ancestors of the Irish.)
Samhain was the new year of the Celts. They did believe that the walls between this world and the otherworld grew thin at this time, but then, they also believed it grew thin at Beltane. It was a feast to memorialize the dead, but it was also a harvest festival, the last of the year before the coming of winter; it is no mistake that the Welsh name for this feast--Nos Galen-gaeof--translates as "Night of the Winter Kalends"--the beginning of winter.
Important events: