For me, day one used to be handing out the syllabus and getting right into whatever the first topic of the course was. I taught a lot of calculus 1, so usually I'd open with a motivating example for the concept of a limit.

After a couple of years, I decided to instead spend day one on the most common egregious gaps in knowledge, so day one was how to add, subtract, multiply, and divide fractions, and then a little practice solving simple rational equations of a single variable.