No, celebrating Halloween is not a sin. Those who assert that it is have the burden of substantiating their claim with Scripture or with Church documents.
Can Catholics celebrate Halloween?
Many non-Catholic Christians worry about Halloween because they appear to believe that the secular traditions associated with the holiday (e.g., spooky decorations and costumes, trick-or-treating) connote approval for evil. For Catholics the spirit of Halloween is best summed up by St. Thomas More
The devil, the proud spirit, cannot endure to be mocked.