Utter sh**te

Rating: 3/10 (21 October 2017)

First of all, the premise alone is so unbelievable. Suppose it were legal to commit crimes during a 12 hour window once a year. Who'd be dumb enough to risk to go outside, even if you hated someone so bad you wanted to kill them? Basically, it's 12 hours of lawlessness, yet the movie still tries to convey that people will have SOME loyalty. E.g. most will kill poor people but not in their own communities. But would you really trust anyone except your family during these 12 hours?
Second, it assumes that crime will drop from it. That is completely unrealistic and ignores all crimes of passion. But yeah, those people will just think: No wait, there'll be a day where I can do this legally, let's calm down and wait!". Crime would never go down. Battery, homicide, etc. would still happen.
Third: This premise is not new at all. It's basically how it was in Europe in the middle ages with the feud "system". If the "purge" was real, it would be the same. Suppose person A kills person B during purge. Family of B would probably seek revenge the same night or next purge. It would escalate between families. Until the government would just say "F it and let's establish the old system again". There's a reason that we have the law system we have today and not the one from the middle ages. A purge is utterly nonsense.
That leads to the next point: This purge would be way too inefficient. Imagine the clean up they'd have to do. Police, Hospitvals, Fire Departements etc. would be way busy with the aftermath of it. And what about all the people killed who had jobs? Businesses would probably have to search for new employees after every purge.