Perche

The strange thing about Carnevale is that it seems to be mostly a weekend thing. As Obscure2k said, it is mostly europeans coming down for the weekends. During the week you see people walking around in costume, but it is a little bit busier than before or after Carnevale. During the weekend everyone floods in, mainly from other areas of Europe, and it gets packed. During the beginning or the end is a matter of choice. It kicks off with some type of princess, usually an actress, flying down what is like a bungee cord from the clock tower in Piazza San Marco. It ends with a big fireworks display over the water near San Marco. There are events pretty much every night.

"Carne" is the word for meat, both in Italian and Spanish
"Va" is the word that means go, also in both Italian and Spanish
"Le" is an article, like the word "it."

Carne-va-le means "meat goes away." It is a celebration for people to party before fasting, meaning no meat for 40 days prior to Easter. Venice extends it for weeks. They even have Carnevale parties a week before Carnevale starts. Because the whole thing lasts a few weeks, but it does peter out during mid week, so if you want to experience it make sure you are there for the weekends. There is something every night, but the weekend is when it really happens, when people from all over come to Venice.

If you can be there at the beginning, the flight of the angel (Volo del Angelo), who is always called Maria bungee cording down from the clock in Piazza San Marco to start it off, that is good, but the final weekend is better. It's more festive, more party like, so I'd lean to that.