How do you ask someone for their date of birth in Spanish?

If you want to ask someone when their birthday is in Spanish you say: "¿Cuándo es *su / tu / vuestrocumpleaños?" (formal singular/informal singular/informal plural)

*"Su" in this case refers to both formal address, singular and plural in both genders:

Su cumpleaños (de usted)

Su cumpleaños (de ustedes).