From the OKC Embark web site:
http://embarkok.com/tools/frequently-asked-questions

3rd Friday Free
One simple but effective way Oklahoma City residents can help improve our community's air quality is to leave their car at home as much as possible during ozone season, and especially on ozone alert days. EMBARK is making it easy for Oklahoma City metro residents to do their part to improve air quality. EMBARK offers free bus rides on all regular routes every third Friday through September.

On Free Fare Fridays, you can still commute, run errands and attend events around the city while doing your part to help your friends and neighbors breathe easier, by simply taking the bus.

Friday, Apri 20th, 2018 is OKC Embark's first free Friday bus ride day for 2018. This intentionally coincides with Earth Day weekend.

I plan on riding tomorrow and several more free Friday's both to save gasoline & to experience the changes the bus system has made since last year. I've always enjoyed riding the bus, both in OKC and around the world. (I just plan on a longer travel time getting from place to place.)

Friday, Apri 20th, 2018 is OKC Embark's first free Friday bus ride day for 2018. This intentionally coincides with Earth Day weekend.

I plan on riding tomorrow and several more free Friday's both to save gasoline & to experience the changes the bus system has made since last year. I've always enjoyed riding the bus, both in OKC and around the world. (I just plan on a longer travel time getting from place to place.)

I ride the bus basically everywhere. I like the fact that my pass can also get me to Norman via the 24 Sooner Express.