What is the earliest date that a taxpayer can file for an automatic extension for returns due in April, 2017? Is there any different policy for 1040 and 1041 accounts?

We expected to find a specified range of dates, but so far have not been able to determine if there is one. We would guess that the last day to file an extension is the date a return would normally be due. This would be useful information now and for later years.