The below little code snippet will help you to do that by
using DayRender event of Calendar control.
.ASPX
<asp:Calendar ID="Calendar1"
runat="server" ondayrender="Calendar1_DayRender">
</asp:Calendar>
CodeBehind
protected void Calendar1_DayRender(object
sender, DayRenderEventArgs e)
{
if (e.Day.Date.DayOfWeek ==
DayOfWeek.Monday)
{
e.Day.IsSelectable = false;
}
}