My answer is definitely no. It is the same like trying to lose extra weight via going to the gim once per year. Of course it is better than nothing, but It should be acknowledged that one day couldn't change anything. From the other hand, one day is a done action...the first step, if you wish. I don't really think that there is any one challenge solved without the first step. As a conclusion, I want to repeat myself, only one car-free day is not a solution,but the direction to the right side, what probably even much important.