<%@ Import Namespace="System.Drawing" %>
<!DOCTYPE html>
<html>
<head>
<script  runat="server">
Sub ChangeDayHeaderStyle(obj As Object, e As EventArgs)
      cal1.DayHeaderStyle.ForeColor = Color.FromName("Blue")
End Sub
</script>

</head>
<body>

<form runat="server">
<asp:Calendar id="cal1" runat="server">
<DayHeaderStyle ForeColor="#FF0000" />
</asp:Calendar>
<asp:Button id="button1" runat="server" OnClick="ChangeDayHeaderStyle"
Text="Change DayHeader Style" />
</form>

</body>
</html>