Graphic via "Prorogation: How can the government suspend Parliament?" BBC
Edit: I kept misreading the calendar as August for some reason, apologies.
The order authorising prorogation states that:
It is this day ordered by Her Majesty in Council that the Parliament be prorogued on a day no earlier than Monday the 9th day of September and no later than Thursday the 12th day of September 2019 to Monday the 14th day of October 2019
Hence prior to that date, Parliament may sit.