For the next two days, plus some additional hours depending on your time zone, Rocket League is free for grabs. It will remain so until this Sunday, April 24th. The Steam offer ends at a different time in different time zones. For PST time zone the time is 1 pm and for EST time zone the time is 4pm. Also, Steam has another special offer for the Rocket League players. You have a 40% discount on the game. The offer ends on April 25th.
Visit this link to get more info and play the game.