assuming you want it all to be considered for those programs that offer earlier interviews, probably late july/early august is best. It gives you enough time to get your score back by the time you submit ERAS on the first few days of September. For the rest of the programs, you could push it later to late august/mid-september but that seems to be cutting it close (plus you'll wonder whether you could have gotten better interviews if only your Step 2CK score was back)
my rec: late july/early august