The traditional Irish pronunciation of Samhain is "sow-in," with the "-ow" sounding like the "-ow" in "cow." Other cultures pronounce the word differently; for example, Gaelic pronunciations include "sow-een" and "shahvin."
While a native English speaker may be tempted to pronounce Samhain as "samhane," there is no linguistic foundation for this pronunciation.
The word "Samhain" is Irish Gaelic in origin and means "summer's end." Samhain takes place on October 31 and is commonly called "Hallows" or "Halloween" in the United States. The Celts created Samhain and celebrated it as the time of year when the worlds of the living and the dead were the closest.