The earliest observation of Halloween was as a Celtic celebration of appreciation for their ancestors.

Samhain, which extended from 31 October to 2 November, was a time the Celts set aside to free themselves from the constraints of their normally highly structured society.
The people would show their lack of inhibition by wearing strange clothing and playing pranks, while children would knock on neighbors' doors asking for treats.
These activities, however, were not central to the celebration.
What was most important to the Celts was the belief that on these days contact could be made with departed spirits, and guidance or inspiration received.
Therefore, the dead were not feared, but celebrated as loving guardians and guides.
Ultimately, the days of Samhain were renamed by early Christians to be Hallowe'en (31 October), All Hallows [All Saints Day] (1 November), and All Souls Day (2 November).