For me, day 1 was by far the most difficult because I was very hungover and I had to deal with it on my own rather than start drinking to get rid of the headache. I had to count the minutes until sleep came.
Day 1 was also extremely difficult because that voice in your head tells you that you have nothing to lose. Why should today be day 1 when tomorrow can be day 1 just as easily? Or the day after that?
Any day can be a day 1 but day 2 felt like a rare treasure. Not that day 2, 3, and beyond were not difficult, but they felt like they had some substance, like I had already accomplished something .
Good question!