I realize this is an old post, but in case of future questions/looking for answers -- Carnaval is determined by the start of Lent -- anyone in the US can figure it out by first finding Ash Wednesday on a calendar for that year. The official date of Carnaval is the Tuesday before Ash Wednesday; most celebrating will begin the Saturday previous, with even smaller celebrations the week prior as well (i.e., the Saturday before the Saturday before...).
The date changes annually, much like Easter, etc. but it is always the week of what we know in the US as Ash Wednesday.
Hope that helps! (also, more or less the same as Mardi Gras.....)