Samhain is observed on October 31, 2017. Samhain is a Gaelic festival marking the end of the harvest season and the beginning of winter or the "darker half" of the year. Most commonly it is held on 31 October - 1 November, or halfway between the autumn equinox and the winter solstice. Along with Imbolc, Beltane and Lughnasadh it makes up the four Gaelic seasonal festivals. It was observed in Ireland, Scotland and the Isle of Man.
Kindred festivals were held at the same time of year in other Celtic lands. Samhain is still celebrated as a cultural festival by some (though it has mostly been replaced by Halloween) and, since the 20th century, has been celebrated as a religious festival by Celtic neopagans and Wiccans. Neopagans in the Southern Hemisphere often celebrate Samhain at the other end of the year (~30 April – 1 May). Some Neopagans celebrate it at the astronomical midpoint between the autumn equinox and winter solstice (or the full moon nearest this point).
In the Northern Hemisphere, this midpoint is when the ecliptic longitude of the Sun reaches 225 degrees. Samhain is mentioned in some of the earliest Irish literature. Many important events in Irish mythology happen or begin on Samhain. It was popularized as the "Celtic New Year" from the late 19th century, following Sir John Rhys and Sir James Frazer. (With material from: Wikipedia)