<p>Does "postmarked by September 31st" mean:</p>
<ol>
<li>You can still mail whatever you are mailing on the 31st and that "postmarked by 31st" simply means the last day you can mail it out?</li>
</ol>
<p>or</p>
<ol>
<li>You must mail whatever you are mailing X days before the 31st, and the "postmarked by 31st" tells the mailman to get it to whatever place by the 31st?</li>
</ol>
<p>You are enabled to mail it up until September 30th (or possibly October 1st).</p>
<p>When you go to the post office, your package will be stamped with the day’s date on it.</p>
<p>Post office? What if I just leave the envelope outside my house before the mailman comes to drop/pick up the mail and he takes it? Will that delay anything? And should I play it safe by doing so the day before considering I will be giving my letter through my home, not the post office?</p>
<p>When you are obligated to “postmark” something, I believe that the person wants you to go to the post office to have it stamped. Perhaps someone else can confirm that though.</p>
<p>Also, I recommend finding out if the person meant September 30th or October 1st.</p>
<p>If you’re going to be handing the letter to your mailman, it’ll probably get to your post office anyways, then get postmarked, then get transported. So I’d play it safe and try as early in advance as possible, or at least a day or two before.</p>
<p>^Yes, I just found the following:</p>
<p>[Does</a> “postmarked by” include the date listed? - Yahoo! Answers](<a href=“http://■■■■■■■■■■■■■■■■■/question/index?qid=20100618235136AAXjyD3]Does”>Yahoo | Mail, Weather, Search, Politics, News, Finance, Sports & Videos)</p>