There are certain rules which every tax payer in this country need to abide.
You can file return after the due date also.
However, in case you have some tax due to be paid to the government, you will have to pay penal interest @ 1% per month for late filing on Tax Payable amount.
Apart from due date, there is last date of filing the return also. Last date for filing of any IT Return of Financial Year, will be last day of next to next financial year. For example, in case you have not filed your Income Tax Return for the financial year 2013-14. The due date for this will be 31st of July, 2014. And last date of filing will be 31st March of 2016.
Beyond which the return will become time barred and you would not be able to file the same. Assessing Officer may require you to pay penalty up to Rs.5,000 for late filing of Income Tax Return.