You should only measure your temperature once per day – in the morning, as soon as you wake up – and this is the temperature you log into the app.
The basal body temperature is your resting temperature and is at its lowest when you sleep, and your basal body temperature is only static for a short time as you wake up from a deep sleep which is why you need to measure while lying down and as soon as you wake up in the morning. You can learn more about basal body temperature here.
Why can I set an evening reminder to measure?
The evening reminder is there to help remind you to prepare for measuring the next morning, for example, by putting your thermometer next to your bed.
It will also remind you to log any additional symptoms you may have experienced during the day (for example, bleeding, cervical mucus, LH tests, etc.) if you wish. Remember that all the data you add is for that calendar day, meaning 24 hours from midnight to midnight.
If you don't want to receive a reminder in the evenings, turn this off by going to the menu in the top-left corner of the app > Settings > Reminders. You can also choose which reminders you'd like to get, at which times, and on what days.