I don't know if they will do it again this year, I haven't seen confirmation, but an awesome event at Lassen National Park the past few years has been the Vehicle Free Day -- the day before the road through the park opens, they allow bikes, peds, scooters, etc. but no motor vehicles. It is pretty awesome. The road is free of cars for 27 miles each way, from Manzanita Lake on the north to Kohm Yah-Mah-Nee Visitor Center to the south, a nice 54 mile round trip with some pretty good climbing. They even did it once in the fall a couple of years ago when the snow held off long enough to actually be able to plan the closure.
The biggest problem is it is hard to plan in advance because they don't announce it very far in advance of road opening -- they don't know for sure when the road will open until it is near time to open. Last year it was on June 11 (a June 12 road opening for cars, but it wasn't announced until June 3. Since 2010, the latest the road opened for cars was July 16 (in the snow year of 2011) and the earliest was May 3, in 2015. Going back to 1980 the latest road opening was July 21 (in the heavy snow year of 1995) and the earliest was April 28 in 1990.
Well, it is going to be June or July this year, my best guess for this year is sometime in July, especially since there are plenty of trees to clear this year along with a lot of snow, and I hope they have another Vehicle Free Day. It really is a great way to enjoy the road through the park.