"When and where do we celebrate carnival in the Netherlands?"
Translation:Wanneer en waar vieren wij in Nederland carnaval?
It is celebrated right before lent, which starts 40 days before Easter on on Ash Wednesday. This often happens in February, but since Easter occurs on different days it can also take place at the beginning of March.
The day it happens heavily depends on what place you live. Some places start on Friday or have events on Thursday evening, while others don't start until Sunday.