We will not add PIN / password protection to aCalendar.
Adding such a mechanism would make the user think that there is any additional protection while this in fact would not be true, as all other calendar apps on the device would still show the data without such pin or password as all calendar data in Android is available to any app with the calendar permissions (and granting permissions to apps or installing apps from Google Play does not require to enter any password).
PIN/password protection should always be set up in your Android security settings to globally protect your phone.