There will be no classes on the following official Dutch holidays:
New Year’s day
Easter Monday
Kingsday
Liberation day (Once every five years; in 2020, there will be no classes on Liberation Day)
Ascension day
Pentecost (Whit Monday)
Christmas
Please check this page for specific dates on each of the aforementioned holidays: https://www.rijksoverheid.nl/onderwerpen/schoolvakanties/vraag-en-antwoord/officiele-feestdagen