(I don't understand why if you put it in the actual calendar, you then have to type a post??)
I suppose the people who wrote the Calendar feature figured more often than not people would want to provide a bit more info for their events than would fit on the calendar itself.
I just finished reworking this for us so that now we don't have to make a post on this board if we'd just like to add something to the calendar. The default setting is that this board is selected to include a post, but if you click on the arrow to the right of the board's name when posting an event to the calendar, you'll see the "Post No Topic" option. Highlight it, click "Post Event", and you're done.