It is the feast of Pentecost. If you want to know more, you can look:

http://en.wikipedia.org/wiki/Pentecost

I'll Just Post this:

Public Holiday

Since Pentecost itself is on a Sunday, it is automatically a public holiday almost everywhere. Additional restrictions to Sunday opening hours may apply.

Pentecost Monday is a public holiday in many European countries including Austria, Belgium, Cyprus, Denmark, Estonia, France, Germany, Hungary, Iceland, Luxembourg, the Netherlands, Norway, Portugal, Romania, and (most parts of) Switzerland. In Sweden it is also a public holiday, but Pentecost Monday (Annandag Pingst), through a government decision December 15, 2004, was abolished and replaced with the Swedish National Day. In Italy and Malta, it is no longer a public holiday. It was a public holiday in Ireland until 1973. It was a bank holiday in the United Kingdom until 1967.