There’s another way to do this and it gives the bonus effect of integrating the GNOME Clock App (with the calendar) that sits in the GNOME notification area and normally is only accessible from the Evolution calendar module.

Assuming you’ve already pointed Rainlendar2 at Evolution’s default *.ics calendar … Bring up the Add-ons dialog within Thunderbird and have it search for the Evolution Mirror extension. Install, restart, done. Now you can update your Thunderbird/Lightning calendar and have it automagically updated in the GNOME notification area and in Rainlendar2.