Originally Posted by
Algorithm 
Living one day at a time comes from the serenity prayer, but not drinking just for today (but perhaps tomorrow) is such a reprieve for the addiction that it instinctively latches on to that idea, grateful to still have a chance to drink every day, despite all the trouble it has led to.
Not drinking "just for today" is a daily reprieve -- for the addiction. This is precisely why people promote that idea. Heartcore's post about requiring lots of re-evaluations and having a giant bag of sobriety chips going back to 1986 is a disheartening example of this.
Good points Algo. Living Life one day at a time might well come from the serenity prayer, which itself is a lot older than AA. It has been solidly incorporated into the AA 24 hour plan set out in steps ten and eleven, about page 84 85 ish, where we review the day gone by, and in the morning, make our plans for the day ahead.
There is nothing about waking up and deciding not to drink just for today. Drinking is not mentioned as, through the earlier steps, the problem has been removed. Instead it is all about how to live the coming day successfully.