You can theoretically wear the sensor during your period, but you don't have to. The first day of your period marks the first day of your cycle.
We recommend using the trackle from day 5 of your cycle onwards in order to get enough measurements for an evaluation – that way you can also conveniently wear a cup or tampon during your period. If you want to wear the trackle during your period, you can only do so in combination with a sanitary pad or period underwear.