In the USA, the due date is April 15. It is not extended to the 16th. If you owe money and if you postmark on the 16th, you will be one day late and will owe one month's worth of penalties.

If you do not owe, or are entitled to a refund, the due date is still April 15 - but the penalty is calculated as a percentage of what you owe and you would owe no penalty because you owe no tax.