The fourth quarter 2013 estimated tax payment deadline is Wednesday, January 15th. If you need to make an estimated payment, it must be postmarked on or before the 15th. If you use EFTPS, the payment must be made on or before the 14th; it takes one day for EFTPS payments to post.
If you mail your payment, I strongly urge you to use certified mail, return receipt requested. That gives you proof of mailing–something that is quite useful if your mail goes astray.