Recently I told a colleague that I'd get him my review by Thursday. By that I had meant by the end of the day on Thursday. However, as I thought about it, he might have been expecting it at the end of the day on Wednesday.
"I'll get it to you by Thursday" is already an admission that something is late, so what difference can one more day make? That extra day is Friday and nobody is going to take a Friday deadline seriously because you are not going to take the weekend off to deal with the thing. Hence, Monday.