The Spanish letter essential in the standard spelling of “quinceañera” is ñ.
The word is spelled quinceañera, with an ñ before the final era. In Spanish, ñ is a separate letter of the alphabet and represents a palatal nasal sound, similar to the sound in the middle of the English word “canyon.” It is not simply an n with an optional accent.
The tilde developed historically from a shortened form of n written above another n in medieval manuscripts. Spanish eventually adopted ñ as a distinct orthographic character. This history is why the letter appears in many familiar Spanish words, including año, meaning year.
Replacing ñ with n can make the word difficult to identify correctly and may change pronunciation. In informal English-language text, people sometimes write quinceanera when keyboard access is limited, but quinceañera remains the standard Spanish spelling. The acute accent in some related forms and the ñ are separate features of Spanish writing.