If the 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 to defines the title for the event's comment. Use the properties like: ToolTipDelay, ToolTipPopDelay and ToolTipWidth properties to specify the 

The tooltip supports the foolowing HTML tags:

• <b>  bold </b>
• <u> underline </u>
• <s> strikeout </s>
• <i> italic </i>
• <fgcolor = FF0000> fgcolor </fgcolor>
• <bgcolor = FF0000> bgcolor </bgcolor>
• <br> breaks a line. 
• <solidline> draws a solid line
• <dotline> draws a dotted line
• <upline> draws the line to the top of the text line
• <r> aligns the rest of the text line to the right side

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

Private Sub Form_Load()
    With CalendarCombo1
        With .Events.Add(Date)
            .CommentTitle = "Just a title"
            .Comment = "This is a bit of text that should appear when the mouse is over the date.<br><r><dotline><upline><b><fgcolor=0000FF>Exontrol ExCalendar</b></fgcolor>"
        End With
    End With
End Sub