April 17 is one of six free days offered every year in the park system
April 18 is Volunteers In Parks (VIP) Day, which highlights the important roles of volunteers in the parks
April 19 is Military Monday, an appreciation of active-duty and veteran members of the military
April 20 is Transformation Tuesday, exploring how the parks have changed over the years
April 21 is Wayback Wednesday, featuring the way parks preserve history and culture
On April 22, the NPS joins the rest of the world in celebrating Earth Day

https://amp.travelawaits.com/2563380/california-national-parks-events-national-park-week/

The City Nature Challenge is an international effort for people to find and document plants and wildlife in cities across the globe. It’s a bioblitz-style competition where cities are in a contest against each other to see who can make the most observations of nature, who can find the most species, and who can engage the most people.

More Info
https://citynaturechallenge.org/

https://m.youtube.com/watch?v=yO45mO9BUJM&t=39s