Who The Wikipedia Day is an event organized by the association Wikimedia CH for Switzerland. Wikimedia CH is an nonprofit association. The second article of its charter states:

The purpose of the Association is to support the creation, collection and distribution of Open Content in a not-for-profit way in order to support education and the equal opportunity of access to knowledge. In terms of the Association, Open Content refers to all work that has been placed by its author under a license, which allows everybody to modify and distribute these works. In addition, the awareness of the related social and philosophical issues will be enhanced.

It is the Swiss representation of Wikimedia Foundation Inc.

Therefore behind Wikipedia, to defend its purposes, to release them and to represent them, there is the Foundation with various local representations.