I am not quite sure if Wikipedia Day is a synonym for establishment day of Wikimedia CH. I would rather say, first of all we should establish Wikimedia CH.
We have to do logical activities.
Maybe I missed a thread, but at the moment it seems to me as we would like to do just a Wikipedia Day in august. I did not find an establishment day of Wikimedia CH yet.
First of all we need a society. Then we can show, what this society does. This would be the Wikipedia Day in august.
I recommend to establish Wikimedia CH in may. May is the end of spring, everybody is going to marry in may. Then in august we throw the party - it will be the Wikipedia Day in august. It may be a kind of birthday party.