<p>in my case, some of my letters were done while my professor was on vacation, even out of the country</p>
<p>I will say if they don’t submit their letters on or 1-2 days before the deadline, won’t hurt to remind them. (just be sure he is not the don’t-need-your-reminder-I-know-what-I’m-doing type of guy)</p>
<p>Like Professor X said, the letters are not done by you, so they don’t reflect yourself, no need to worry too much about too much. Letters won’t make any difference in the first few levels of screening anyways. If the department reaches the point need to see your letter, they will contact you for it (which they did to me). They will not do any harm to you due to LoR absence.</p>