Since 2003 every year in March some institutions in Bern opens their doors for a night and show their interior to the public. First it has started only with museums (that's because it's called "Museum Night"), but then also other institutions like banks, hotels, parliament building, townhall, and different services joined the event.
All the buildings that are involved, are illuminated, so you'll find it easily.
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=> Homepage Museumsnacht Bern