The Origins of Halloween

How did these ancient Celtic practices come to America?

When the potato crop in Ireland failed, many of the Irish people, modern descendants of the Celts, emigrated to America bringing with them their folk practices which were remnants of the Celtic festival observances.

We in America view this as a harvest festival. Did the Celts also view it as such?

Yes. The Celts had 3 harvests. Aug 1, or Lammas, was the first harvest, when the first fruits were offered to the Gods in thanks. The Fall Equinox was the true harvest. This was when the bulk of the crops would be brought in. Samhain was the final harvest of the year. Anything left on the vines or in the fields after this date was considered blasted by the fairies ("pu'ka") and unfit for human consumption.

Does anyone today celebrate Samhain as a religious observance?

Yes. Many followers of various pagan religions, such as Druidism and Wicca, observe this day as a religious festival. They view it as a memorial day for their dead friends and family, much as the mainstream US does the national Memorial Day holiday in May. It is still a night to practice various forms of divination concerning future events. It is also considered a time to wrap up old projects, take stock of one's life and initiate new projects for the coming year. As the winter season is approaching, it is a good time to do studying on research projects, and also a good time to begin handwork such as sewing, leatherworking, woodworking etc., for Yule gifts later in the year. And while "satanists" are using this holiday as their own, this is certainly not the only example of a holiday (or even religious symbols) being "borrowed" from an older religion by a newer one.