Our modern Halloween festival is really an American invention which takes the Christian festival of All Saints Day (or All Hallows) and takes its more ghoulish appearance from Mexico's festival Dia de los Muertos which is a three day festival and sometimes more than three days, depending on local traditions (and exuberance).
Halloween is often mistakenly thought to have its origins in the Celtic festival of Samhain. This is not true because the Celtic tribes of these islands, of Hibernia and Caledonia, left no written records. The only written records come from the Romans 2000 years ago and they are not exactly reliable or unbiased.
Some people have claimed that Samhain was actually a Celtic god. In fact there is no convincing evidence to support this. It seems likely that this is a misinterpretation of Celtic paganism by those of a theist persuasion. And the word 'pagan' is itself also the subject of wild speculation.
The word comes from the Latin 'paganus' which was used to describe country dwellers; then, as now, city dwellers regarded those in the countyside as ignorant yokels. The Roman influence in the UK has been long lasting.
Samhain was absorbed first by the Romans into their Feralia, a festival of the dead, and also with their harvest festival in honour of Pomona. This merged Roman festival was itself incorporated by the Christians and rebranded as All Saints Day, leaving the night before to become all hallows eve, hallows e'en, thus Halloween. https://en.wikipedia.org/wiki/Samhain
It was a standard practice of many early religions, especially the Christian church, to take local customs and places of power and co-opt them into their own belief system. This was probably one of the earliest known examples of the "embrace, extend and extinguish" strategy that (unfortunately) is so commercially successful today.
You can forget about any of those 19th century inventions of Druidry or Paganism or witchcraft, all of which claim to be a direct lineage from the past but are, in reality, based more on the Romantic movements of recent European history.
Samhain has survived in the oral traditions and the music of the Celtic tribes.