cross-posted from: https://fedia.io/m/Thunderbird/t/1140808
Plan Less, Do More: Introducing Appointment By Thunderbird - The Thunderbird Blog
Thunderbird has a new project under its wing: Appointment. Learn all about our approach to appointment scheduling, and try it yourself.
I think this is a scheduling app, i.e. for other people to try to get things into your calendar (which you can also view in Thunderbird).