What does the line-crossing ceremony commemorate?
Correct answer: Person's first crossing of the Equator

Doug Moore
I am a proud Golden Shellback. crossed the equator 1982 on board USS Enterprise and again 1984 same ship.

Clidge
Doug Moore, for me it was the USS Hornet as part of the Apollo 11 recovery crew, 1969.

antzuzu
Delta flight from Atlanta to Rio de Janeiro in 2015 they performed an "equatorial baptism" on two crew members - The reason I knew the answer (tho, traditionally it's done on a ship, obvi.)

arohanui
I crossed the equator as a toddler and again as a teenager.