It must be post marked by midnight tonight to be considered filed on time. You may want to file an extension, instead, so that it will be considered filed on time. Any tax balance due should be paid or mailed tonight.
For more information, see:
How do I file an IRS extension (Form 4868)?
When are Taxes Due? Important Tax Deadlines and Dates - TurboTax Tax Tips & Videos
What if I missed the May 17 filing deadline?
@atalarsb
