prefectural citizens' day
A designated day celebrating the residents of a specific prefecture, often with events or free admission to facilities.
Today is Prefectural Citizens' Day, so the prefectural art museum is free to enter.
Compound of 県民 (けんみん, 'prefectural citizens') + の (possessive particle) + 日 (ひ, 'day'). A straightforward modern coinage for a civic observance.