Hi Estanito,

Thank you for report.
That issue was fixed. Update will be available within coming service releases.

For now you may get around that issue by forcing repainting calendar on client. One option is to repaint calendar explicitly on ClientEvents.Initialize. Another option is to use dummy ClientEvents.RenderDay. Below are examples:

<script type="text/javascript">
function WebMonthCalendar1_Initialize(sender, eventArgs)
{
 sender.repaint();
}
</script>
<ig:WebMonthCalendar ID="WebMonthCalendar1" runat="server" HideOtherMonthDays="True">
  <ClientEvents Initialize="WebMonthCalendar1_Initialize" />
</ig:WebMonthCalendar>

or

<script type="text/javascript">
function WebMonthCalendar1_RenderDay(sender, eventArgs){}
</script>
<ig:WebMonthCalendar ID="WebMonthCalendar1" runat="server" HideOtherMonthDays="True">
 <ClientEvents RenderDay="WebMonthCalendar1_RenderDay" />
</ig:WebMonthCalendar>