Peak Day is typically the most fertile day of your cycle, and is named as such because it denotes your day of peak fertility. Your Peak Day is the last day that you have fertile (i.e., eggwhite or watery) cervical fluid or a wet or slippery vaginal sensation in a cycle. Peak Day generally occurs a day before or the day of ovulation (which means that Peak Day generally occurs 1-2 days before the temperature shift).
After your Peak Day, your cervical fluid will go back to being creamy, sticky, or dry. If you don't have any eggwhite or watery cervical fluid in a cycle, your Peak Day is the last day of the wettest-quality fluid that you have - so if your wettest cervical fluid is creamy, your Peak Day would be the last day of creamy fluid before it goes back to being sticky or dry.
So, your Peak Day is the last day of either:
- Wet, stretchy, or slippery cervical fluid
- A wet, slippery vaginal sensation
- Any midcycle spotting (midcycle spotting is usually due to ovulation, and is actually very fertile fluid)
Because you need to wait and see if your cervical fluid and vaginal sensation dry up the next day, it's only possible to determine the Peak Day in retrospect. Keep in mind that the Peak Day is not necessarily the day of the greatest quantity of fertile cervical fluid, but rather is the last day of any quantity of fertile cervical fluid or a wet vaginal sensation. It's possible for the greatest quantity of fertile cervical fluid to occur before the last day of fertile cervical fluid.
If you are charting to avoid pregnancy, knowing when your Peak Day is allows you to apply the Peak+4 rule of the Fertility Awareness Method.