you ever wondered why so many on the left use GNOME? let me tell you why, it's because of libadwaita, it's liberal approach that lets an app be suitable for any device. and guess what it actually stands for, LiberalAdwaita. Yep.
@julia it's all webdav stuff so you can just use davx5 and link it to an app that supports that, like i use fossify calendar (github.com/FossifyOrg/Calendar)
(edit: i realised that the other calendar isn't maintained so i found the maintained version of it)