When do I have to complete my tax return?

The filing deadline is April 15, 2019.

However, you can request an extension to file your return by October 15.  [This is an extension of the time to file, NOT a time to pay any taxes due.]

Tax returns are due April 15, 2019.  
Any Tax Due payment is due by April 15, 2019

If you get an extension (or even if  you don't) you can efile until Oct 15, 2019.  After that you will have to print and mail it.   If you are getting a refund you have 3 years to file.