It depends on how long your luteal phase is. Generally it's around 14 days, so 14 days before your af is due then you would ovulate. If you have really regular cycles then it's easy to work out. If you have a 28 day cycle then you would ovulate on day 14, so DTD on the couple of days before and the day of ovulation. For me, i have long irregular cycles. This past month i ovulated on day 26 and had a 11 day lp. If you want to pinpoint it then you can check your cm, temps and do an OPK. I don't bother with OPKs though because they can get expensive. I just temp and check my cm.