Open Data Day is a gathering of citizens in cities around the world to write applications, liberate data, create visualizations and publish analyses using open public data to show support for and encourage the adoption of open data policies by the world's local, regional and national governments.
The international activities on March 5th are gathered here: http://opendataday.org.
In Amsterdam, UvA students organize an Open Data Day on 'Amsterdam in the Golden Age' at the Amsterdam Museum, from 12.00-17.00 hrs.