This includes but isn't limited to:
The birthday events are by far the most numerous, and each one has an attachment with the villager's picture as well as a link to their entry on nookipedia.com for more information.
In addition, each event has a similar link to be able to read up further.
Note that not every Animal Crossing event is marked in the calendar, as not all of them have predictable dates.
This has been created as a Google Calendar - if you use a Google account you can subscribe to it directly, or if you use another calendar app you can add the iCalendar link instead.
The calendar is rich with villager birthdays:
Note the attachment, Bluebear.png - there's one on every birthday so even if you don't recognize the villager by name, you can quickly see if you know them by appearance!
This is an AC:NL fishing tourney date - it's very likely to be different in AC:NH!
Some events are impossible to set up through a normal calendar app and had to be set up through the API - not super interesting, but something I'm proud of. Google Calendar doesn't support having an event repeat every <day> of a single month every year.
All events are set up in such a way that the calendar will continue to be correct for years to come.
If it interests you, this repository also contains the tool I wrote to set up all the hundreds of birthdays.