I don't know if that works as intended.
People who are conscious about the problem already try to use as little petrol as possible. I have a car, but try to use it only when necessary, i share it and take passengers over long rides. But i need to buy fuel now and then.
So, when i don't buy it on september 22nd, i'll buy it one day before or after, where's the point?? The amount would be the same.
Fuel is not as easy to boycott as a certain brand or other product, like tofu, for example (i could eat something else instead). If i want to boycott fuel, i'd have to go for alternatives rahter than limiting myself to only buy fuel on fridays