When the summer began, it felt as if we’d all be humming Green Day’s “Wake Me Up When September Ends” through the final month of baseball’s regular season.

The MLB postseason in October, and likely first-round byes under the new 12-team playoff format, seemed like a lock for much of the first half for the Yankees and the Mets as they built double-digit division leads.

Now that Labor Day — and with it, unofficially, summer — is behind us, sudden divisional pennant races for both first-place local clubs only will add to an already busy September viewing schedule amid the New York sports landscape.

Here are some key dates to mark down for the remainder of the month, including the most important series for the Yankees and the Mets.