On Samhain and Halloween

 My go to on stuff like this is Ronald Hutton's The Stations of the Sun. Which is a semi-scholarly study of the entire British ritual year. He is generally reliable but when he strays from British or Irish primary sources, he is less so. (For example; on Christmas he take Syrus Scriptor at face value)

He concludes that Samhain was definitely a pre-Christian festival, but what we know is sketchy. It definitely was:

* the final summer's end festival with games and feasting for up to 3 days before and after

* a period of heightened spiritual activity, but not connected to the dead. 

* possibly, but not definitively, the beginning of the old Irish year. (The Roman calendar was introduced with Christianity in the 4th century, the first records of Samhain are post 11th century.)

As for Halloween, he points out that the original Irish date for All Saints followed the Roman observance on May 13, and the Nov. 1 date was introduced via Germany. The connection with the dead seems to be with the Nov. 2 All Souls commemoration which was introduced in 998 by Odilo, Abbot of Cluny and migrated to Nov. 2 around 1200.