ovulation is when your body releases an egg and that can take only a few hours. The egg that is released will last only 12-36 hours after that.
If that egg is not fertilized by sperm then it deteriorates and you get your period.

so you see, ovulation would happen about that same day that the calendar predicts and you are most fertile two days before and one day after that ovulation date.

Only by recording your temperatures first thing in the morning before getting out of bed (using a Basal Thermometer) will you pin point your real ovulation dates.

Your temperature sign is the sign that will best help you to precisely pinpoint the day that ovulation occurred. This is because progesterone, and hence your temperature, increase quite dramatically just after ovulation has taken place.

Before ovulation, there is only a small amount of progesterone present in your body and your basal body temperatures (your resting temperatures) are in the lower range. After ovulation, when there is increased progesterone secreted from the corpus luteum, temperatures become elevated. The temperature elevation that occurs after ovulation is sufficient to be measured with a BBT thermometer and viewed on a BBT graph when a few simple conditions are met.

The rise in temperature is usually about 0.4 degrees Fahrenheit or 0.2 degrees Celsius, but the rise may be as slight as 0.2 degrees Fahrenheit or 0.1 degrees Celsius or even less in some cases. The actual temperatures are less important than noting a biphasic pattern showing two levels of temperatures. FertilityFriend.com will indicate that ovulation has taken place once a sustained temperature rise is observed (at least three days).

If there is no pregnancy, your temperature will stay elevated for 10-16 days, until the corpus luteum regresses. At this time, unless there is a pregnancy, progesterone levels drop dramatically and you get your period.

Ovulation occurs when a mature egg is released from the ovary, pushed down the fallopian tube, and is available to be fertilized. The lining of the uterus has thickened to prepare for a fertilized egg. If no conception occurs, the uterine lining as well as blood will be shed. The shedding of an unfertilized egg and the uterine wall is the time of menstruation.

The first part of the ovulation cycle is called the follicular phase. This phase starts the first day of the last menstrual period (LMP) and continues until ovulation. This first half of the cycle can differ greatly for each woman lasting anywhere from 7 days until 40 days. The second half of the cycle is called the luteal phase and is from the day of ovulation until the next period begins. The luteal phase has a more precise timeline and usually is only 12-16 days from the day of ovulation. This ultimately means that the day of ovulation will determine how long your cycle is. This also means that outside factors like stress, illness, and disruption of normal routine can throw off your ovulation which then results in changing the time your period will come. So the old thought that stress can affect your period is only partly true.

The timing of ovulation is complex and can take some studying of your body and your cycles to figure out. By using a combination of methods such as studying cervical fluid, taking your body temperature, and tracking your periods, you can calculate your time of ovulation. The American Pregnancy Association encourages women to learn about the fertility awareness method of tracking cycles and then combine that with ovulation predictor kits to best understand when you are ovulating. The Association estimates that this is anywhere between 11-21 days since the last menstrual period (LMP), or 12-16 days from when you expect the next menstrual period to start.

Ovulation itself actually occurs very quickly, less than a day. Often time people refer to ovulation when they actually mean a woman's fertile period. Unfortunately, timing this is more of an art than a science.