Bem-vindo(a) ao carnaval!
Here's a breakdown:
* Bem-vindo(a): This is the formal welcome, meaning "Welcome." It's gender-specific, so "Bem-vindo" is for a male, and "Bem-vinda" is for a female.
* ao carnaval: This means "to the carnival".
You could also use the informal "Bem-vindos(as) ao carnaval!" which is more common in a casual setting.
If you're in Brazil, where carnival is a huge celebration, you might hear people say:
Seja bem-vindo(a) ao Carnaval! (Welcome to Carnival!)
Enjoy the carnival!