sì infatti, sia con la union che con la where sono riuscita a risolvere, grazie 1000 , non era nemmeno complicato

codice:
SELECT Mid(CStr(DateTimeCall),1,10) AS Data, Count(Mid(CStr(DateTimeCall),1,10)) As Chiamate FROM Chiamate WHERE (DateTimeCall Between #7/6/2007# AND #7/19/2007#) 
AND (((weekday(DateTimeCall) <> 1 AND weekday(DateTimeCall) <> 7) AND (TimeValue(DateTimeCall) Between #08.30.00# And #12.30.00# OR TimeValue(DateTimeCall) Between #13.30.00# And #18.30.00#)) 
OR (weekday(DateTimeCall) = 7) AND (TimeValue(DateTimeCall) Between #08.30.00# And #12.30.00#))
AND GroupID <> "Telefoni" 
GROUP BY Mid(CStr(DateTimeCall),1,10)