So Far, Both Leagues Have Had Their Highest OPS In September And That Is Somewhat Rare

Here are the years it happened in the AL since 1914, including this year (using the Baseball Reference Play Index-it also includes any regular season October games):

1914	0.662
1916	0.671
1918	0.768
1924	0.765
1973	0.722
1982	0.740
1985	0.750
2015	0.759

Now the NL

1939	0.733
1968	0.652
1973	0.714
1985	0.720
2007	0.784
2015	0.743

So 14 times out of 202 possible years (101 for each league, not counting 1994 when there were no games in September). So it happens about 6.9% of the time. This will be only the third time for both leagues in the same year. The AL came close in 2007 with a .770 OPS in September while May was highest with .771.

The second highest month in the AL so far was August with .739. In the NL, it is also August with .734. So the AL is a much better bet to have its highest OPS in September, since that month is currently .020 ahead of the next highest while it is only .009 higher for the NL.