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.
