It will happen at block 420000. Bitcoin tries to maintain it's block frequency at an average of 10 minutes in the long term but due to increase in hashrate during the midst of the difficulty term, it is impossible for it to be at 10 minutes. However, we can more or less tell where it is. It will happen at approximately 2016-07-22 UTC. Of course, if the blocks gets delayed at 419999 due to varience, it would be pushed back slightly.