Red, white & blue
I think it is safe to say that Dutch people aren't that flag minded. Unlike for instance US citizens, a lot of whom have at least one stars and stripes in their garden (as was my experience when visiting the Chicago area). There are just a few holidays that seem get the Dutch to hoisten the flag en masse. Of course there's Queen's Day last Monday, but also the commemoration of war victims on May 4 (half-staff) and the festive Liberation Day on May 5.
And on a more personal basis: people hoist the driekleur ("triple colour") when their kids pass their highschool exam ;-)
