You ovulate around mid cycle and then you get your period around 2 weeks AFTER ovulation. So you need to be having sex around mid cycle, i.e. ovulation.
There are many ways you can test for ovulation, but using OPKs are the most accurate way. If you don't want to use them, then just start having sex every other day (not every day) from around cycle day9/10 onwards until your almost due your period - that should cover all your bases.
I believe its exactly two weeks after Ur menstrual period or when u ovulate like on Ur ovulation days