order by event date not by post date

I would like to make posts which make up an agenda (by category ‘agenda’). It can be that I add a post on day “a”, for an event on day “m”. And the next day (“b”) I maybe want to add an event for day “g”. The posts will be ordered “a, b”, but that means that the order of the events will be day “m” and then “g”. And what if I want to add an event the next day (“c”) which will take place on day “k”? The order of the posts will be a, b, c, but the order of the events will be “m, g, k”
So I’m looking for a way to add the event date to a post and make WordPress use THAT date to order my events. And without having to edit any of my templates.