I haven't heard of 'Buy Nothing Day' before but it is an interesting concept. I am not sure what affect this would have on consumer driven economies. An idea I like is to set aside a period of time in which you don't buy anything that wasn't around in a certain period. eg For a week don't buy anything that was not available before 1970. This makes me think about what I take for granted, what 'stuff' I think I really need, and stops impulse buying.
I will be interested to hear more about 'buy nothing day'.