I think you've pointed me in the right direction. thanks!
the link itself does not seem to work, though. I've tried putting the text in, and I've also tried the following:
HyperLink link = new HyperLink();
link.Text = e.Day.Date.Day.ToString();
link.NavigateUrl = CMSContext.CurrentAliasPath + ".aspx?date=" + Server.UrlEncode(e.Day.Date.ToString());
e.Cell.Controls.Add(link);
I've even tried to put test text in there - no joy. it just shows the day number. no errors, it just gets ignored.