It depends on your
cycle. Generally, you ovulate about 14
days before your
period. So, if you have a standard 28 day cycle and the first day of your cycle was on the 14th, you would ovulate (be fertile) the 28th.
However, many women do not have a standard 28 day cycle. To figure out your fertile time, you can track your cycle and use an ovulation predictor kit. There are also other signs (basal body temp, cervical mucus) that you can look for.