Liberation Day
[noun]
[de be-vrij-dings-dag, de be-vrij-dings-da-gen]

Wondering why the Dutch national flag has been put out everywhere today? It’s Liberation Day: the day that the Netherlands were freed from German occupation: May 5th, 1945. It was the end of World War II in the Netherlands.On May 5th, there are festivities everywhere, and there are a number of so called "bevrijdingsfestivals", which are usually free open air concerts where Dutch pop/rock artists perform in front of large audiences.

Liberation Day is preceded by "Dodenherdenking" (Remembrance of the Dead), every year on May 4th. Not only the fallen soldiers and victims of the Second World War are remembered, but the Dutch lives lost in every war and peace mission the Netherlands were involved in. During "Dodenherdenking", the flags are flown at half-mast.

Examples:
– "Vandaag is het Bevrijdingsdag! Heb je de vlag al uitgehangen?" 
("Today is Liberation Day! Have you put out the flag already?")

– "Vanwege Bevrijdingsdag, zijn er vandaag overal bevrijdingsfestivals." 
("Because of Liberation Day, there are liberation festivals everywhere today.")

Related words:
– Vrij: free [adjective].
– Bevrijden: to free, to liberate [verb] [bevrijdde, bevrijd].
– Bevrijding: liberation [noun] [de bevrijding, de bevrijdingen].
– Dag: day [noun] [de dag, de dagen].

Extra:
There’s always a bit confusion whether "Bevrijdingsdag" is an official holiday or not. Well…it is an official holiday, but not everyone has a day off. The Dutch government has decided that employers and employees should come to an arrangement themselves, which effectively means that people working for the government, public organizaions etc. get a day off, and people working for commercial organization usually don’t get a day off…