Full Calendar claims to be “The Most Popular JavaScript Calendar”. I don’t know for sure that that’s true but I wouldn’t be surprised. It’s quite full featured and produces very nice results. I’ve been using it since 2015 to include course calendars on my class webpages. That was version 2 and it had jQuery as a requirement. FullCalendar has developed a lot since then and, with the release of version 6 last month, it’s now easy to import it into Observable dynamically using:
FullCalendar = import("https://cdn.skypack.dev/fullcalendar")
Here’s an example: