Start wearing the Ava as soon as you receive it, no matter where you are in your cycle. The most important thing is to wear the Ava consistently every night for your entire cycle, as the information collected from a whole cycle helps the algorithm improve its accuracy for your predictions.
If you start wearing the Ava during your follicular phase and you haven't reached your estimated ovulation date, then Ava will start working immediately to detect your fertile window in real-time.
If you start wearing the Ava during your luteal phase and you have past your estimated ovulation date, then Ava will use the date of your last period to estimate when your most recent fertile window occurred.
As soon as your next period comes, ensure that you enter those dates into the app accordingly - Ava will begin looking for signals to detect when your fertile window is opening.
For more information on how to add or edit a period, please take a look here.