The end of August/beginning of September.
I am due the 27th. Conceived on sept 4
Every woman is different, we have different cycle lengths and not everyone has a 14 day luteal phase (time between ovulation and the next period), that's why I say end of August/beginning of September. Unless you know EXACTLY when you ovulate each month, it's hard to say when for sure. MOST women ovulate 14 days before their period, so if you know when you were due to start, count back 14 days and that's about when.
If paternity is called into question DO NOT attempt to guess at who the father might be. You need to have a DNA test done when the child is born. If it's not, when conception happens does not matter.
I have the I'm expecting app on my phone and it says the date I apparently conceived. I'm sure you can find something on the internet :)