Ovulation refers to the time during a woman’s menstrual cycle when eggs are released from the ovaries to the fallopian tubes. It is also one of the times when women are most fertile. Calculating ovulation is an important step for many women as it can guide them in effective birth control.

In addition, women can also determine their ovulation period through the following methods:

1. Observe changes in body temperature: Women’s body temperature will change significantly during the cycle. During ovulation, a woman's body temperature will rise by about 0.3-0.5 degrees Celsius. You can determine your ovulation period by recording changes in your body temperature.

2. Observe vaginal secretions: During ovulation, a woman’s vaginal secretions will become moister and brighter in color. You can determine your ovulation period by observing your vaginal discharge.

3. Use David test strips: David test strips are a commonly used method to test a woman’s ovulation period. It can determine ovulation by measuring HCG levels in a woman's body.

To sum up, determining a woman's ovulation period requires understanding her menstrual cycle and determining it by observing body temperature changes, vaginal secretions and David test strips. If a woman is unsure about her menstrual cycle or how to use David test strips, talk to her doctor or professional fertility counselor.

So, if your period comes on the 18th, when will you ovulate? Generally speaking, a woman’s menstrual cycle lasts 28 days. Therefore, if your period comes on the 18th, your ovulation period will be around the 25th. However, specific dates may vary based on individual differences. Therefore, if a woman is unsure about her menstrual cycle or how to use David test strips, please consult a doctor or professional fertility counselor.