Hi Hasithi,

WebMonthCalendar does not support persistance of custom days modified on client.
There is array of those days, so, that array can be modified and any item can be removed. However, you should build logic to find a day which you want to modify. Please look at docs for get_customDays and what members it contains. You also may put debugger and check those members (days).

Below example will remove day at index 2.

 <script type="text/javascript">
function removeCustomDay()
{
 var cal = $find('<%=WebMonthCalendar1.ClientID%>');
 var days = cal.get_customDays();
 //debugger;
 var idToRemove = 2;
 if(idToRemove >= days.length)
  return;
 days[idToRemove] = days[days.length - 1];
 days.length--;
 cal.repaint();
}
</script>
<ig:WebMonthCalendar ID="WebMonthCalendar1" runat="server">
  <CustomDays>
      <ig:CalendarCustomDay Day="5" CssClass="red" />
      <ig:CalendarCustomDay Day="8" CssClass="red" />
      <ig:CalendarCustomDay Day="9" CssClass="red" />
      <ig:CalendarCustomDay Day="15" CssClass="red" />
  </CustomDays>
</ig:WebMonthCalendar>
<input type="button" value="clear" onclick="removeCustomDay()" />