After you ovulate
In the average 28 day cycle you will generally ovulate on day 14 or between day 11-16.You can get get pregnant only after you ovulate. After you ovulate there are only 24 hours for a sperm to fertilize that egg. If the egg is not fertilized then the egg will no longer be viable. If the egg is fertilized then it will need to implant in the uterus. Once it implants in the uterus it will burrow in and grow. Your hormones will change and you will be able to get a positive pregnancy around the time your period is due.
Some women have longer, shorter, or even irregular cycles. Track your cycle so you can learn what to expect from your body. Some women also ovulate early or late.