How do you say happy 21st birthday in spanish?

quinceañera has no English Word , but quince is fifteen and añera would be a girls years

now

that's as close to explaining what it means

so what it is, is a celebration on a girls 15th birthday. much like a sweet 16 party in the U.S.

difference is Parents usually go all out for this one like they would for a wedding.

yet

you asked for

la quinceañera which is what you call either the girl or the party