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;

}

}