I'm flying Delta POP-ATL-POP in early August & when making the reservation a month or so ago it was already unavailable after the 10th (so I'm flying before then).
However, though there weren't flights later in August... I was able to book my return flight POP-ATL in mid/late December for the holidays. I'm hoping I can find a Delta flight back to POP after Christmas - so much nicer not having to connect in Miami.