I was going through the Starfinder APs pages, and noticed a weird pattern in the expected release days.
September: Attack of the Swarm 2 and Attack of the Swarm 3
October: Attack of the Swarm 4
November: Attack of the Swarm 5
December: No books
January: Attack of the Swarm 6
It feels like Attack of the Swarm 3 to 5 were accidentally moved one month up. Not sure if this is intentional or by accident, just wanted to bring this up just in case it is an accident.
The only release date given for those products is the September 18th for Attack of the Swarm 2. All of the other dates are preorder dates, which are generally a few weeks earlier than the street dates.
If you check this page, you will see that the actual release dates after August are indeed monthly. The lack of a December preorder date occurs because we go from an early in the month shipment for December (listed as a November preorder) to a late in the month shipment for January (listed as a January preorder).