XFCE? so something really lightweight like Orage probably. Which is in fact part of XFCE…

What’s stopping you from using the default calendar app of your operating system? It does all the things you mention, be it MacOS, Linux (Gnome, GTK etc) or Windows. What do you need that the default thing can’t do?

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Maybe the Danish don’t just count with their fingers to 10, but include their toes… So 10 fingers + 10 toes = 20?