The length of the luteal phase (What is a luteal phase?) is usually constant or varies by only one or two days. This also applies to irregular cycles, which means that even if the length of your cycle varies from cycle to cycle, the time between ovulation and the next menstruation is usually constant. So, for example, if the average length of your luteal phase is 13 days, and if your May cycle was 26 days long, then ovulation probably occurred at around the 13th day of the cycle (26 minus 13). Similarly, if your cycle in June was 35 days long, then you may presume that ovulation occurred at around the 22nd day of the cycle (35 minus 13). Of course, these are only approximate values, so if you want to if know ovulation has occurred at all, and if it has – when, use more reliable methods, such as measuring your basal body temperature or try USG monitoring.