As a practical matter—is it better/easier/more advisable to upgrade to Calendar 2 before upgrading to ExpressionEngine 3, or the reverse?
You cannot upgrade to Calendar 2 while on EE2, as it is only compatible with EE3. We created a very thorough walkthrough and user guide for customers wishing to upgrade from Calendar 1 / EE2 to Calendar 2 / EE3 here:
So to exactly answer your question, you need to upgrade Calendar and EE at the same time (take site offline, upload EE3 files, upload Calendar 2 files to EE3 addons folder, then upgrade EE. Then upgrade Calendar, and update your templates). :)