A lot of the standard histories of Halloween that you will find on the Internet, or on television shows like the History Channel's
The Haunted History of Halloween, are badly researched, and keep repeating a lot of incorrect information about Halloween. Every modern day Halloween practice or tradition is always attributed all the way back to the ancient celts, although in fact, much of our "tradition" is recent.
For instance:
• There is little primary documentation  in Ireland, the UK, or the US  of costuming or masking on Halloween before 1900.
• From the 17th century to the 20th century, the term "jack-o'-lantern" in Britain referred to the will-o-the-wisp swamp lights phenomeon, not to carved vegetable lanterns. Americans in the early 19th century were the first to apply that term to carved vegetable lanterns  in our case, pumpkins.
• In Britain, there is no evidence that carved vegetable lanterns were associated particularly with Halloween before about 1920. Meanwhile, in North America, carved pumpkins had been associated with Halloween since the mid-19th century.
• Trick-or-treating in America is probably unrelated to any British or Irish antecedent like the medieval practice of "souling". Ritual begging on Halloween was virtually unknown in America before the 1930s, and did not become a widespread custom until the 1950s.
More on these topics from Wikipedia:
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Jack-o'-lantern
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Trick-or-treating