Via Deutsche Welle:
Since 1990, October 3 has been a holiday in Germany to mark the reunification of the country. But there is another tradition that falls on October 3: it's also the day of a nation-wide open house at Germany's mosques.
The first mosque open house took place in 1997, and this year, the motto is 'The Quran - 1400 years, topical and part of everyday life.' The purpose is to celebrate the revelation of the Quran to the Prophet Muhammad 1400 years ago.