As SouthernMama08 said, ovulation is not exact. Our bodies are not clocks.
Ovulation usually occurs 11-16 days prior to your next period; you're most fertile in the 2-3 days before that. Many women have the same luteal phase (time between ovulation and next period) every cycle, but not all. For example, a woman with regular, 28-day cycles, may ovulate on day 14 every month; likewise, a woman with very irregular cycles may have a period anywhere from 25-50 days, but always a 13-day luteal phase. However, there are women (like me) whose luteal phase changes every month; "normal" for me is 10-14 days, averaging 12 days. There can also be occasional cycles when ovulation doesn't occur, even in the most "regular" of women.
In a nutshell--unless you chart and know when you ovulate, there's no way to tell. Once your period starts, you can count backwards to get a rough estimate, but that's all it is.
at 2:28 PM on Apr. 11, 2011