Therefore if any person is [ingrafted] in Christ (the Messiah) he is a new creation (a new creature altogether);
the old [previous moral and spiritual condition] has passed away. Behold, the fresh and new has come!
(2 Corinthians 5:17 AMP)

The world celebrates Halloween by dressing in costumes as monsters or other characters and pretending to be a different person for a few hours.
Christians celebrate life by actually becoming a new creature in Christ Jesus for all of eternity.