We get an extra day because of the holiday so you can postmark by the 16th. Don't forget to send in your checks! It sucks parting with a big chunk of change, but what are you gonna do.
Ummm... what are you talking about? Est. taxes aren't due until April 15th. "Your first estimated tax payment for each year is due April 15th. You may pay the entire year's estimated tax at that time, or you may pay your estimated tax in four payments." http://www.irs.gov/taxtopics/tc355.html
No, that would be for Q1 of 07. The payment due today is for the Q4 of 06. http://www.irs.gov/publications/p505/ch02.html#d0e6371
if you missed the 15th you still can avoid penalty by filing your tax return by the end of January 31