Well, for me it's quite good practice (buy nothing day). Cause nowadays everyone can see plenty of everything. People buy some goods and items spending much money. I guess, most of often we buy unnecessary things.
As for pro and contra: (advantages)
Firstly, you don't have to spend money. There will be more important situation, when you can do it.
Secondly, you can use your time to develop yourself. Learn English, do sport or travel instead of buying doubtful thing.
As for disadvantages, I hardly ever can say anything about it, cause I ve got everything in today's abundance.
As for disadvantage..
Well, I 'm so used to gettig, having, buying everything daily . So , I almost haven't got "buy nothing days" in my life, this in its turn is my disadvantage.