I like to separate the menstrual cycle into 4 parts. The first part is menses and should be fairly consistent from cycle to cycle. After menstruation is the follicular phase where your body prepares for ovulation. The length of the follicular phase can vary from month to month, which is what makes your longer or shorter some months. The follicular phase ends with ovulation, which is followed by the luteal phase. The luteal phase is where implantation would occur, and the length of the luteal phase is consistent from cycle to cycle, so once you confirm that ovulation has occurred you will know when your period will arrive.
There are several ways to track your cycle to know when you’ve ovulated. The book “Taking Charge of Your Fertility” as well as the tutorials for the app Fertility Friend provide a very good explanation of what signs your body gives you that ovulation is approaching. Cervical mucus, cervix position, and ovulation prediction kits can give you an idea that your body is gearing up for ovulation, but the only way to confirm ovulation (besides being monitored by ultrasound) is to track your basel body temperature. After ovulation there is a sustained rise in your basel body temperature, caused by progesterone.
This past cycle I had a positive ovulation test and fertile cervical mucus leading up to cycle day 17, and then my temp rise on cycle day 18, confirming ovulation on cycle day 17. My luteal phase is 10-11 days long so I expected my period on cycle day 27 or 28. I usually have a temperature drop the day my period arrives. Cycle day 27 arrived and no temp drop. Cycle day 28 and no temp drop. Cycle day 29, no temp drop. I took a test on cycle day 29 and it was negative. I was spotting so I was confident that I was not pregnant, but I couldn’t be sure until my period arrived. I was sure of my ovulation date and 3 days late and had a glimmer of hope that maybe, just maybe, something had finally worked, but no, 4 days after my period was expected it arrived. Cycle 23 now.