By default, the Selectable property is "not(eventdisabled)", meaning all non-disabled dates are selectable. Set the Selectable property before updating the selection, as changing it does not affect the current selection. Use the SingleSel property to specify whether the control allows single or multiple selections. The control fires the SelectionChanged event when the user selects a new date. The SelDate property selects or unselects a date. The UnSelDate method unselects a date.

The Selectable expression supports the following keywords:

• value, represents the date being checked
• isevent, indicates whether the date being queried (value) has any event associated with it (added via the Events.Add method)
• eventuserdata, refers to the UserData property of an event added through Events.Add method (UserData is a custom value or object you can associate with an event for storing additional information, such as IDs, tags, or any metadata relevant to your application)
• eventdisabled, refers to the Disabled property of an event added through Events.Add method (Disabled is a boolean property that indicates whether the event is inactive or unavailable. You can use this to restrict actions or display based on the event's enabled/disabled state)

This allows you to define restrictions based on:

• the date itself (value)
• the date has any event associated
• the state of the event (eventdisabled)
• the event's user-data (eventuserdata)

This property/method supports predefined constants and operators/functions as described here.

For instance: 

• "not(weekday(value) in (0,6))", makes all dates selectable except Sundays and Saturdays)
• "not(month(value) = 7)", prevents selecting dates in July, all other dates are selectable
• "not(month(value) = 1 and weekday(value) in (0,6))", prevents selecting Saturdays and Sundays in July, all other dates remain selectable
• "not(isevent)", specifies that only dates with no event associated are selectable
• "not(value in (#1/1/2001#,#1/2/2001#))", disables selection for January 1 and 2, 2001; all other dates are selectable
• "not((value format `MMdd`) in (`0101`,`0102`))", prevents selecting January 1 and 2 of any year while allowing all other dates