Learn Spanish (Latin America): Su cumpleaños es el tres de octubre.

In Spanish, you use normal numbers for dates, like el tres (the three). The only exception is the first of the month, which uses the word primero (first).

Why is it el tres de octubre and not just "octubre tres"?

To say a date in Spanish, the required pattern is el + [number] + de + [month]. You always need el (the) before the number, and de (of) before the month.

Why isn't the month octubre capitalized?

In Spanish, months of the year and days of the week are always written in lowercase unless they are the very first word of a sentence.

Cumpleaños ends with an 's'. Is it a plural word here?

It looks plural, but cumpleaños is singular in this sentence. It is a compound word made of cumple (completes) and años (years). Because it already ends in an 's', the singular and plural forms are exactly the same: el cumpleaños (the birthday) and los cumpleaños (the birthdays).

The translation says "His birthday", but could su cumpleaños mean something else?