The former Roma defender has made over 30 appearances for Gianfranco Zola's side this season, part of a team that is within two points of the npower Championship automatic spots with eight games to play.
Cassetti, currently on loan from Udinese, is enjoying his first season in English football and focused on getting the Hornets into the top flight.
He told the club's official website: "The fans at Watford are fantastic because at home or away we have a lot of people following us. They are very important.
"We want to give them a present at the end of the season. With eight games to go, this is a very important moment of the season."
He continued: "Every week there is a surprise, this is what I like in this Championship. It's incredible, you must be fit every match, with good concentration because if not you are dead."