If that property is not set, then default calendar is used. That default calendar is shared between all WebDatePickers on page.
Notes:
The drop-down calendar on client cannot be used for actions which will conflict with default behavior of WebDatePicker. For example, set_selectedDate(date), setFocus() and similar actions are not supported.
If calendar is located directly in the Form, or in the same container(s) where the WebDatePicker is, then the ID property of calendar can be used. Otherwise, WebDatePicker will not be able to find calendar on server. Therefore, application should use ClientID property of calendar and it should ensure following style attributes
display:none;
visibility:hidden;
It is not recommended to set that property after the Page.OnLoad event. Also to ensure shared calendar the local EnsureSharedCalendar() and static WebDatePicker.EnsureSharedCalendar(page) methods can be used.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2