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The Christmas season in the Philippines is being compared to the Christmas season in other countries in the world. This means the adjective needs to be in superlative form since:

Long: Positive form- used to describe one thing
Longer: Comparative form- for comparing (only two)
Longest: Superlative form- for comparing more than two things 

Therefore the answer is longest and the sentence should be:
Christmas season in the Philippines is perhaps celebrated the longest in the world.
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