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. The SelectionChanged event occurs when the user selects a new date. The SelDate property selects or 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