One day or day one (Part 1)

One Day” means some day in the future. “One day I’ll visit Paris.” The deeper meaning though is that it’s never going happen; it’s wishful thinking. The person waiting for one day is not doing anything to make it happen. “One day” never comes.
“Day One” means the first day of a plan or figuratively, “What’s the first thing you are going to get done.” 

E.g., “Day one, we hike in and set up base camp. Day two, we scale the peak and return to base camp. Day three, we take down base camp and hike out.” It means you have specific goals and accomplishments and a way to achieve them.
So the phrase means you can continue with wishful thinking that is never going to get you what you want or you can start planning and acting to accomplish what you want.