Mountain Day
A Japanese national holiday celebrated on August 11, established to appreciate mountains and nature.
Mountain Day is on August 11th.
I plan to go mountain climbing on this year's Mountain Day.
山の日 (August 11) is specifically about mountains, whereas みどりの日 (May 4) is a broader holiday for appreciating nature and greenery in general.
Compound of 山 (yama, 'mountain') + の (no, possessive particle) + 日 (hi, 'day'). The holiday was established in 2014 and first observed in 2016.