They say that the most fertile time is mid way through your cycle .. so the 10 days prior to your period is a good time to get jiggy.
Also, any time you are well pished and forget to use contraception rofl. xxx
More specifically, it is around day 14 (with day 1 being the first day of your period) when most women are ovulating and therefore most fertile. It does vary between women though, and even between cycles on the same woman, but in general from around day 12 to around day 16 is the most fertile time.
If you REALLY want to know you can buy ovulation predictor sticks that work on your saliva so you know when you're going to be fertile, or chart your temperatures to help you predict it.