Where does the word “Halloween” come from?

The name is derived from Old Irish and means roughly "summer's end".
The word Halloween is first attested in the 16th century and represents a Scottish variant of the fuller All-Hallows-Even ("evening"), that is, the night before All Hallows Day. Up through the early 20th century, the spelling "Hallowe'en" was frequently used, eliding the "v" and shortening the word. Although the phrase All Hallows is found in Old English (ealra hálȝena mæssedæȝ, the feast of all saints), All-Hallows-Even is itself not attested until 1556.

Information: http://en.wikipedia.org/wiki/Halloween