Hi, if her cycles are irregular, simply have sex every third day when she is not having her periods. Sperm can live 5 days in her body, so you will have covered all the possibilities if you have sex every 3 or even every 4 days. Ovulation comes two weeks before the next period, so it is not predictable except looking backwards from the period, which of course is too late by then. Especially in a woman with irregular cycles, ovulation does not come reliably two weeks after the last period. So do what I suggest and have sex on a regular basis but not so often that it depletes your sperm count. Every 3 or 4 days is fine.
Beginning of September...but without knowing how long her cycle is or if it's regular/irregular it's hard to say