Back or leg day.
If you want to do just "hang" cleans. Go for it on back day. However, if you're doing power cleans, you may want to try it on leg day.
Leg day, they involve more hamstring, glutes and quads more than upper body muscles.....you will feel them in the traps though, but stick them on leg day and do them when you are fresh.
A 'hang' clean is one that starts with the bar resting on your thighs just above your knees. From there you only dip slightly w/ the legs to generate the momentum to get it to your shoulders. There is no deadlift portion. You're doing power cleans so yeah, I'd probably throw it on leg day too.
I generally do them w/ my back. So many muscles in the upper back are utilized by this technique. And unless you are doing the true olympic style power clean the legs are not terribly activted.