prefectural citizens' day
A designated day celebrating the residents of a specific prefecture, often with events or free admission to facilities.
今日は県民の日だから、県立美術館が無料で入れる。
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.