Look on the mailbox itself for the pickup times. If the latest pickup time has already occurred, then the mail inside the box will sit there until the next scheduled pickup.

Since the 15th is on a Sunday this year, if I were you and the envelope HAD to be postmarked by the 15th, I would either mail it today (14th) or take it to the main U.S. Post Office wherever you live.

I found a website called "The Mailbox Locator". Here's the introductory blurb:

"The Mailbox Locator helps you find USPS collection stations (blue mailboxes) and post offices in your area. Click on a state name or enter a zip code to see a list of US Postal Service blue mailboxes and post office locations in that area. Every listing contains location information (most with maps) and scheduled pick-up times for each mailbox. For buzzword compliance this site also includes links to mailbox map matchups showing multiple mailbox and post office locations for most U.S. zip codes."

http://www.payphone-project.com/mailboxes/

Or you can go to the US Postal Service website post office locator: http://www.switchboard.com/usps.1355/dir/6_0/index.htm

It all depends on when it hits the processing center. Your best bet is to go to the post-office(or a UPS store or Mailboxes, etc.) and mail it there, as it will be post marked right there in front of you.