Correct spelling


Correct spelling, explanation: the word anniversary comes from the Latin word annus (genitive anni), meaning a year. That is why we spell anniversary with double n, not one. Aniversary is a misspelling because the origin of the word point to double n.

Definition of anniversary:
noun, annual recurrence of an event, a celebration of an important past event
We are going to Greece for our wedding 1st anniversary.
Major gave a speech during the anniversary celebrations.

Incorrect spelling


Incorrect spelling, explanation: aniversary is the wrong form because anniversary should be spelled with double n due to its roots. Latin word annus, which means year, and is a core word for anniversarium is spelled with double n, just like later, English, anniversary.