Carnival is a Catholic festival that happens over 3 days, usually just before
Lent, in February or March. Lent is a 40 day season of prayer and fasting that begins on Ash Wednesday and ends on Holy Thursday.
Because traditionally many people go on a fast during Lent, giving up meat, sugar or other foods and drink; carnival is an opportunity to enjoy these treats for the last time.
Carnival is celebrated in many Catholic countries all over the world.
This is also the reason why carnaval is only celebrated in the Catholic South of the Netherlands (N-Brabant and Limburg)