TIU (Tone It Up) does a workout challenge often but the bikini series one is a once a year thing. I am less likely to really do a challenge for working out because I have my own pace, but I am going to try the 2016 bikini series. If you are interested in joining them and the others going in for this you can find more information on their site here.
To get a look at one of the videos from the last series you can go here. You can sift through their page for more free workout videos if you want.