The celebration most folks think of as Carnival ( as in Rio or Trinidad etc) is celebrated on a different date each year because the timing is tied to Easter.
Here on AC it is from Sunday through Tuesday before Lent (which starts on Ash Wednesday). I believe those dates are14-15-16 February in 2010.
Belize celebrates another "Carnival" in a much bigger way in Belize City with amazing parades, costumes etc. This carnival is part of the September Celebrations. Most of the big stuff for this "carnival" happens between the 10th and the 21st of September.