Learn Spanish (Latin America): ¡Felicidades! Hoy es tu cumpleaños.

Cumpleaños is a compound word made of cumple (completes) and años (years). Because "years" is plural, the word always ends in "s". It is singular here, which is why we use tu (your) instead of the plural tus. If you wanted to say "birthdays", the word stays exactly the same: los cumpleaños.

Why do we use es instead of está for "is" here?

We use ser (the verb that es comes from) to talk about dates, days, and time. Since a birthday is a specific date and event, we use es. You will always use es when saying what the date or day is, like hoy es lunes (today is Monday).

Why doesn't tu have an accent mark in this sentence?

In Spanish, tú with an accent mark is the word for "you" (the person). Without the accent mark, tu is the word for "your" (owning something). Here, it means "your birthday", so it does not get an accent mark.

Does Felicidades literally mean "Congratulations"?

It is the most common way to say "Congratulations" for personal milestones, but it literally translates to "happinesses". You will hear it used for birthdays, weddings, anniversaries, and holidays.