Historic Mills Across Czechia Open to the Public on May 9th On May 9, mills across the Czech Republic will open their doors to the public as part of Open Mills Day, a nationwide event aimed at bringing historic technical landmarks back into public life. The date is tied to May 13, 1116, when historical records first mentioned a mill sign in the Czech lands. Across the country, visitors will be able to enter buildings that are usually closed, explore their interiors, and see how traditional milling technology worked.