Saunas are very busy during the 4 days of carnival, most of the boys stick around cause they know there will be an extraordinary amount of tourist both foreigners and from other parts of Brazil, with lots of money flowing around. Besides the locals, boys from nearby states come to Rio just for business, cause is the place to be. Boywise is one the best times of the year if not the best, to be in a Rio sauna, be warned that fees are a little higher at this time of the year.
If you are heading to Rio mainly for boys don't lose any minute with bandas and blocos, the music is boring and monotone and they are basically all the same. If you happen to bump into one on your way to a sauna, give it 15 minutes for the exotic side of it, then follow your destiny and lose yourself in paradise.