I wouldn't suspect the penalty will be all that big for being a week or two late. You can probably google it for other people's experiences. From the info below up to 5% for the 1st full month. You can file an extension immediately if it might take you a week or more to get your return together. You can file the extension on line.
A late filing penalty applies if you owe taxes and didn't file your return or extension by April 18, 2017.
This penalty also applies if you owe taxes, filed an extension, but didn't file your return by October 16, 2017.
The late filing penalty is 5% of the additional taxes owed amount for every month (or fraction thereof) your return is late, up to a maximum of 25%.
If you file more than 60 days after the due date, the minimum penalty is $135 or 100% of your unpaid tax, whichever is smaller.
Tip: The late filing penalty can be 10 times higher than the late payment penalty. If you can't pay your tax bill and didn't file an extension, at least file your return as soon as possible! You can always amend it later.
A late payment penalty applies if you didn't pay additional taxes owed by April 18, 2017, whether you filed an extension or not.
The late payment penalty is 0.5% (1/2 of 1 percent) of the additional tax owed amount for every month (or fraction thereof) the owed tax remains unpaid, up to a maximum of 25%.
For any month(s) in which both the late-payment and late-filing penalties apply, the 0.5% late-payment penalty is waived.