Well the tough thing is you need to figure out how many days your cycle is, because you ovulate 12-16 days before your NEXT period. So if you have a 28-day cycle, count back 12 days and you are most fertile between that day and the next 4. The thing is every women is different, so you may ovulate sooner or later than that. But techincally there are only 4-5 fertile days which are in that window I described above. You can also track when you ovulate my checking your daily temps or using ovulation predictor kits. Or like me, I get cervical mucas and I know that is when I ovualte.
If you are planning to get pregnant, you need to track your cycles. How many days are their between your periods? If you have a standard 28 days cycle, then you would ovulate around day 14. That being the case, you would want to have sex somewhere around day 11/12 thru about day 16 or so. You can have sex every day. However, if your partner has a low sperm count you may want to do it every other day so he can build up reserves. If you periods differ from month to month, it will make it a bit harder to figure out when you ovulate. You might also want to check your CM or track your temps for a few months.