The history calendar which will soon appear on the main page (if it's not there yet when you read this) was thought up by the editorship and started by my comrade Forte who solved the mechanics of how we would make it. I only finished it as he moved on to fixing important bugs and left the calendar dormant.
The way the calendar works is through Template:History calendar.
This template simply calls the "magic words" (what mediawiki calls global variables) that represent a month and day to create URLs.
Example: History calendar/08/14 is the subpage for August 14 (07 being the 7th month, July, and 14th day).
The history calendar template checks the current month and day and pulls the content that's on the corresponding page. It's based on server time which is GMT.
Tomorrow, it will pull /08/15, then 08/16, etc.
Adding events in the calendar
First, go on this page: Template:History calendar. Then, add to the URL the /month/day -- with a leading 0 for single-digit months and days (like 06, 03, 09, etc).
If you want to add Karl Marx's birthday as an event for example, you would have to go to History_calendar/04/05 (May being the 4th month of the year and 05 being the 5th day of May).
If that page doesn't exist yet, you will have to create it. Since there are 366 days in a (leap) year, we'll eventually need 366 subpages for this template, meaning we probably don't have all of them yet when you're reading this!
To start populating that specific day with an event (say Marx's birthday), create the page which opens the editor. We currently follow this format:
Add the year of the event in bold. Then break the line, and call Template:History calendar/calendar event. Years should be ordered chronologically from earliest to latest (e.g. 1860 comes above 1867 in the list).
This template takes in four parameters: year, month, day, and "event". Event being what you want the line in the calendar to say, e.g. "Karl Marx was born".
We don't use year month and day yet, but it starts to populate semantics so that if we change how the calendar works, we have some repository built up of all the events. I figure we'll probably need it some time down the line. So please, when adding an event, do fill in the year month and day in the calendar event template.
We will also use this template to style the events later down the line.
And that's about it! The history calendar template takes care of the rest! (displaying the event you added on the right date etc).
Ideas for later would be to make a full page of all events, which essentially links to every day of the year in the template and pulls the content of these pages as well.