From June 4th to June 21st.
Pokemon fans that live in France are in luck -- the Paris Pokemon Center will be distributing a special Vivillon next month. It sports a unique Pokeball pattern, and will be given out from June 4th until June 21st.
It'll come in at level 12, with Stun Spore, Gust, Light Screen, and Struggle Bug. All you'll need is a copy of Pokemon X or Y. Hopefully the event will hit internationally at some point!
Special Vivillon Distribution [Serebii]