If you filed an extension on your taxes, then you have until October 15th to get the return prepared and sent to the IRS. However, there are several reasons to get the return completed as soon as you are able:
The sooner you get the return done, the sooner you can get it off your mind and in the past.
You don’t want to wait until the last minute to get the information to your tax preparer. There are a limited number of returns that anyone can complete during any one day, and tax preparers usually prioritize their work on a first come first served basis. So don’t expect to be able to bring your return in to them on the 14th and have it prepared on time.
The IRS will shut down e-filing after October 15th to get ready for the following tax year, so any return after that date will have to be sent as a conventional paper return. This leads to the possibility of lost paperwork, and if you have a refund due, will delay your receipt of the money due even longer than if the return is sent via Efile.
If you have outstanding taxes that still need to be paid, the longer you wait the larger the penalty and interest imposed.
If you can’t pay all the tax due it is still better to go ahead and file the return. Only after you have filed the return can you work with the IRS to set up a payment plan schedule with them.