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., egg white or watery) cervical mucus or a wet or slippery vaginal sensation in a cycle (1).
After your Peak Day, your cervical mucus will go back to being creamy, sticky, or dry. If you don't have any egg white or watery cervical mucus in a cycle, your Peak Day is the last day of the wettest-quality fluid that you have. In other words, if your wettest cervical mucus is creamy, your Peak Day would be the last day of creamy mucus before it goes back to being sticky or dry.
So, your Peak Day is the last day of either:
- Wet, stretchy, or slippery cervical mucus
- A wet, slippery vaginal sensation
- Any mid-cycle spotting (mid-cycle spotting is usually due to ovulation, and is considered very fertile)
Because you need to wait and see if your cervical mucus 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 (2). It's possible for the greatest quantity of fertile cervical mucus to occur before the last day of fertile cervical mucus.
If you are charting to avoid pregnancy, knowing when your Peak Day is allows you to apply the Peak+4 rule of the Symptothermal Method.
References
- https://www.tcoyf.com/identifying-peak-day/
- https://billings.life/en/how-the-billings-ovulation-method-works9/the-peak-of-fertility.html