Awaiting an LoR request response
You only got a B- in his class and only talked to him a couple of times during office hours.  You sent mail over the weekend just before a two-day holiday and got an automated response that he was out of town.  It's
 the first day back and you're already worried that he hasn't responded.

In both cases, your expectations are unreasonable.  You didn't do very well in his class and he hardly knows you.  What exactly do you expect him to say that could possibly help your chances of admission anywhere?  He can't just make stuff up, he can only report what he actually knows about you, which is that your performance wasn't that good.  You need to request references from instructors who gave you A's, not B-'s.  And pick people who actually got to know you because you showed up at their office more than just a couple of times and for more than just help on the homework.

When you do ask, allow them time to respond.  Give them at least a week (longer if it's a holiday!) to say yes or no, and then at least a month (more is better!) to write the letter.  (Be sure to tell them the deadline and how the letter has to be submitted.)

You should also provide a set of materials to help your reference write a good letter. It should include a list of schools where you’re applying, copies of your transcript, resume or CV, statement of purpose, and a short reminder of who you are, e.g., the class you took from them, where you used to sit, why they might remember your work or some discussions you had, perhaps a photograph. It’s okay (and helpful) to suggest strengths you hope your reference can speak to, e.g., your creativity, ability for abstract thought, contributions to a group.

Depending on the privacy laws in your country, you may also need to state explicitly, "You have my permission to discuss my academic performance."  For example, here in the US, instructors are prohibited by FERPA from discussing a student's performance without explicit permission.