property Event.CommentTitle as String
Retrieves or sets a value that indicates the event's comment title.

TypeDescription
String A string expression that indicates the title for the for date's tooltip.

If the an Event object has associated a comment or a comment's description, a tooltip appears if the cursor is over the event's date.  Use the ComentTitle property to defines the comment's description. Use the Comment property to define the cell's tooltip information. 

The following sample shows how to attach a comment to "a day before yesterday" date:

Calendar1.Events.Add(Date - 2).CommentTitle = "A comment description"
Calendar1.Events.Add(Date - 2).Comment = "A comment"