If you miss the deadline but pay by December 31, a penalty will be imposed on you. If your income exceeds Rs 5 lakh, then the penalty will be 5,000, and the amount of penalty will be Rs 1,000 if your income is less than Rs 5 lakh.
Besides, you'll be charged an interest of 1 percent every month on your outstanding tax amount. So, in case your tax liability is Rs 10,000 and you miss the deadline, then your penalty will be increasing by Rs 100 every passing month.
Further, if you do not file ITR by December 31, you can face severe consequences.