I am assuming that you are talking of the financial year 2017-18. 

The financial year 2017-18 = The Assessment year 2018-19.

The last date by which you should submit your return (without penalty) is August 31, 2018.

However, you may submit later than August 31, 2018, after paying the penalty.

You have to pay a penalty of Rs5,000 if you deposit before Dec 31, 2018. 

If you deposit your return later than Dec 31, 2018, then you would have to pay a penalty of Rs10,000.

You can't deposit returns after March 31, 2019.

So, in a nutshell:

1. Deposit returns till Aug 31, 2018, with 0 penalty

2. Deposit returns till Dec 31, 2018, with Rs5,000 penalty

3. Deposit returns till March 31, 2019, with Rs10,000 penalty.

Income tax law has one more provision on the penalty.

It states that if your total income is not more than Rs5,00,000, then the penalty charged can't be more than Rs1,000.