The luteal phase is the part of the menstrual cycle which begins immediately after ovulation. Your menstrual cycle begins on the first day of your menstrual period, cycle day one, the first day when you start bleeding. The menstrual cycle ends the day before your next menstrual bleeding begins. In most women the menstrual cycle lasts between 21 and 35 days.

The menstrual cycle is dividided into two phases:

1. The first phase is also known as the "Follicular Phase" and it starts on the first day of your menstrual period, the day your bleeding or period begins, called cycle day one or CD1, and lasts until ovulation.
2. The second phase is known as the "Luteal Phase" and begins on the day after ovulation occurs and continues until the day before your next menstrual bleeding, indicating the start of your next period.  

The length of the follicular phase can normally last between 7 and 21 days, but the luteal phase needs to last a minimum of 12 days to ensure that the fertilized egg has enough time for implantation. Having a normal luteal phase is essential for your fertility.