Strange... When you create the new query don't add any tables or nothing. Click on SQL to open the SQL window and make sure when you paste there is only one ";" at the end of the statement.
I have modified the statement to handle potential problems with Nulls(fields that contain nothing). Replace tblTEST with the name of a table that contains the fields: RecordID, salesdate1,salesdate2,salesdate3,salesdate4,salesdate5,salesdate6
SELECT RECORDID,IIf([salesdate1] Between [Forms]![frmFlash]![StartDate] And [Forms]![frmFlash]![EndDate],[salesdate1]) As Saledate
FROM tblTEST
WHERE Not IsNull(salesdate1)
UNION
SELECT RECORDID,IIf([salesdate2] Between [Forms]![frmFlash]![StartDate] And [Forms]![frmFlash]![EndDate],[salesdate2])
FROM tblTEST
WHERE Not IsNull(salesdate2)
UNION
SELECT RECORDID,IIf([salesdate3] Between [Forms]![frmFlash]![StartDate] And [Forms]![frmFlash]![EndDate],[salesdate3])
FROM tblTEST
WHERE Not IsNull(salesdate3)
UNION
SELECT RECORDID,IIf([salesdate4] Between [Forms]![frmFlash]![StartDate] And [Forms]![frmFlash]![EndDate],[salesdate4])
FROM tblTEST
WHERE Not IsNull(salesdate4)
UNION
SELECT RECORDID,IIf([salesdate5] Between [Forms]![frmFlash]![StartDate] And [Forms]![frmFlash]![EndDate],[salesdate5])
FROM tblTEST
WHERE Not IsNull(salesdate5)
UNION SELECT RECORDID,IIf([salesdate6] Between [Forms]![frmFlash]![StartDate] And [Forms]![frmFlash]![EndDate],[salesdate6])
FROM tblTEST
WHERE Not IsNull(salesdate6);
Give me a shout if you still get errors.