This isn't new. Under the Discover Pass law State Parks must have up to 12 free days.
RCW 79A.80.050
(2) The state parks and recreation commission must provide up to twelve days a year where entry to state parks is free. At least three of those days must be on weekends. When practicable, the free access days should be timed to correspond with any similar free access days planned by the national park service for national parks located in the general region of high volume state parks.
2014 State Parks free days:
•Jan. 19 and 20 – In honor of Martin Luther King Day
•March 19 – In honor of Washington State Parks' 101st birthday
•April 19 – A spring Saturday free day
•April 22 – Earth Day
•May 11 – A spring Saturday free day
•June 7 and 8 – In honor of National Trails Day and WDFW Free Fishing Weekend
•June 14 – National Get Outdoors Day
•Aug. 25 – In honor of National Park Service’s birthday
•Sept. 27 –National Public Lands Day
•Nov. 11 – Veteran's Day weekend