Gisela and I made our honeymoon trip to the Carneval in Rio in 1971. The main event are the parades and the main parade starts late Saturday afternoon until Sunday morning. In 1971 the paade was held on the major avenue Generalissimo Franco (now you should try early to get tickets to the Sambadome).
Beside the parades there is Samba (Carneval) all over the town and for enthusiasts (like me) there are also many Bals (everywhere).
It is one of those lifetime-excperiences I would for sure not miss if possible.