The most common way to say "happy birthday" in Spanish is "feliz cumpleaños". Here are some other ways to say "happy birthday" in Spanish:

• Feliz cumple (informal)
• Feliz vuelta al sol (happy turn around the sun)
• Felicidades (congratulations)

In addition to these phrases, there are many different versions of the "happy birthday" song in Spanish, depending on the country or region. The basic tune is the same as the English version, but the lyrics may vary. For example, in Mexico, the traditional birthday song is "Las Mañanitas," while in Venezuela, it is "Ay, qué noche tan preciosa".

To summarize, "feliz cumpleaños" is the most common way to say "happy birthday" in Spanish, but there are other phrases and songs that can be used as well.