There is no general application deadline for Dutch universities. Applications are already received the year before starting the studies. For many degree programmes, applications are possible shortly before start of study (1. September every year), even in August.
When should I apply?
Favored: December preceding year.
Application without school transcripts is possible. The missing documents can be handed in later. The same applies to evidence of language proficiency.
Studies with registration restrictions
Application by the end of April (e.g. international economy courses at Maastricht University) and 15. May (e.g. medicine)
Studies without admission restrictions
Application by the end of August. Sometimes even a late registration in September is possible.
Contact the university you have chosen for further information on the application and application deadline!