See this New Moon Note on Youtube:

One of my favorite things about learning Fertility Awareness is the end of the confused anxiety, and confused hope, around waiting for a “late period”. This came up twice with my clients this month: One of them got her period 5 days later that she's used to, and was worried that she'd gotten pregnant accidentally. One of them is trying to get pregnant, and her period was 3 days late. She felt hopeful when her usual cycle length had passed and was disappointed when her period started.

So, I reviewed with both of them how there is really no such thing as a late period – there's ovulation that's later than you're used to, and that makes your entire cycle longer than you're used to. If you're charting your cervical mucus and Basal Body Temperature (BBT), you know from the time of ovulation how long your cycle is going to be.

Your cycle has two phases: pre-ovulation, and post-ovulation (also called the luteal phase). The pre-ovulation phase goes from the day your period starts until the day you ovulate. The length of that phase can be as short as a week, but sometimes months go by between someone getting their period and ovulating. The post-ovulation phase, which goes from the day after you ovulate until the day before your next period starts, does NOT vary in length very much. It might change by a day or two during a cycle where you've had some kind of stress or illness or change in health – but it doesn't vary much.

These 3 lines represent 3 different cycles, a short one, a medium one, and a long one. The vertical red line represents ovulation. See how the pre-ovulation phase can be any length, but after ovulation, the number of days until you get your period is set?

This means that if you know when you ovulate, because you're charting your cervical mucus and your Basal Body Temperature, you know when to be ready for your period. And if you're looking to conceive, then it's good to know that seventeen high temperatures indicates that you're pregnant. (A luteal phase can last 9-16 days. Seventeen high temperatures indicates that you are pregnant.)

Okay, so, to recap: there's no such thing as a late period, there's only a cycle that was longer than you're used to. The part of it that's lengthened is the PRE-OVULATION phase, so if you're watching your cervical mucus and charting your BBT, you will be clear about when ovulation happens, and when to be ready for your period.