The letter Ñ, called eñe in Spanish, appears in the word quinceañera.
The ñ is a distinct letter of the Spanish alphabet, not merely an optional decoration over n. In quinceañera, it appears in the syllable -añe-. The tilde changes the pronunciation and preserves the conventional Spanish spelling of the word.
Spanish spelling developed the ñ partly from medieval scribal practices in which a small mark represented a doubled n. The letter later became an important feature of written Spanish and is now used in many Spanish words, including año, “year,” and España, “Spain.”
English-language publications sometimes omit diacritics for practical reasons, writing quinceanera. The fully correct Spanish spelling is quinceañera, however. This detail also helps distinguish año from ano, which are different words in Spanish and should not be treated as interchangeable.