Bellow are two samples from two different women for one cycle.
The ovulation calendars show you how cycles vary from one women to another.

Sample A has a 28 day cycle. Day 1 is the first day of her period. On day 12 she is transitioning from non-fertile to fertile. By day 15 she is ovulating.

Sample B has a 26 day cycle. Day 1 is the first day of her period. On day 10 she is transitioning from non-fertile to fertile. By day 14 she is ovulating.