Day one is the day you get your period. The day before this becomes the last day of your previous cycle. For example, if your period starts on October 6th, that is day one. If your next period stars on the 3 November, the 2nd of November was the 28th day of your last cycle, and the 3rd is the first day of the next one. You count upwards from there until the next period begins - when you start all over again.