Looks like you have an unclosed div or something. I can't replicate the issue on my demo site. For me, passing a null date in the url for a detail page shows the first date of the event by default.
Found the problem it was because the <% control CurrentDate %> is returning null there was a missing </div> because that control wasn't being processed.
Now it looks fine but there is still the issue of it not giving me any date whatsoever, just a blank event page.
Thx for your help so far by the way UncleCheese, it's been very nice.