Here's the code I wrote for you, hope it helps.
Private Sub Command3_Click()
Dim dteStart As Date, _
dteCurrent As Date
' Combobox containing months to check, if nothing set, set it to 1
If Not IsNumeric(cboMonth) Then
cboMonth = 1
End If
' Create the start date for the search, using the month from the
combobox and
' the current year
dteStart = CDate(cboMonth & "/1/" & Year(Now))
' Set the date to search for to the Start date
dteCurrent = dteStart
' Clear the listbox which contains the missing dates
lstDate.RowSource = ""
' As long as the current month is the same as the start month, keep
checking
Do While Month(dteCurrent) = Month(dteStart)
' If the are no records in the table with this date, add it to
the list
If DCount("*", "Table1", "DateEntered=#" & dteCurrent & "#") =
0 Then
lstDate.AddItem dteCurrent
End If
' Get the next date to check
dteCurrent = DateAdd("d", 1, dteCurrent)
Loop
End Sub