Due date for it return

Yes, if Income Tax Department is closed on these days. Otherwise, No.

As the due date i.e, 30.09.2012 is falls on Sunday but All assessees who's Income tax return due date is 30.09.2012 (accounts are required to be audited and corporate assessees ) are required to File return online and online portal is always open and work 24 X 7.

Therefore no question of extending to next working day i.e, 01.10.2012 for corporate assessees.

There is no specific section as such in IT Act. This is general clause in U/S 139.