senti perchè devi settare gli stile in dinamico?

se fai cosi non fai prima:

<asp:Calendar id="cal" runat="server" enableviewstate="false" Font-Name="Arial" Font-Size="12px" TodayDayStyle-BackColor="red" DayHeaderStyle-BackColor="lightsteelblue" OtherMonthDayStyle-ForeColor="gray" NextPrevStyle-ForeColor="white" TitleStyle-BackColor="gray" TitleStyle-ForeColor="white" TitleStyle-Font-Bold="True" TitleStyle-Font-Size="15px" SelectedDayStyle-BackColor="Navy" SelectedDayStyle-Font-Bold="True" onselectionchanged="ShowDate" />