31st July deadline.............

As per the provisions of Section 139(1) the return of income for professional income has to be filed on or before 31st July 2008 in the normal course. If  gross  professional receipts are more than Rs.10lacks then you will be subject to Tax Audit and in such an event the due date for filing the return for the assessment year 2008-2009 will be  30th September 2008. 

As you have not stated what your gross professional receipts are it is presumed that they will be less than Rs.10lacs and in such an event July 31st is the last date for filing the return.  In your case, as you have stated that you have only refund, the dalayed filing of return that is filing beyond 31st July will not attract any penal interest or penalty proceedings. But for getting the refund the return has to be filed at leat before 31st March 2009 other wise you will not be eligible to claim the refund in normal course. Non filing of return beyond 31st March 2009 for the assessment year 2008-2009 will also attract a penalty.