I submitted my application today, but one of my recommenders still did not. I don’t know why it takes him so long, but my plan B would be to have another teacher to submit in place of this one on Monday. Will it be too late to delete this recommender and send email to another one? I have my original recommender name on my application.

@Alamalaki I dealt with this last year, and based on my experience, MIT is extremely lenient when it comes to application materials that are beyond your control. I would send your recommender another email, just as a reminder, and then just wait it out. Even if your LOR is submitted late, they will still look at your application and there will be no penalty to you.