Currently, about 90 million Pokemon have been traded worldwide on Global Trade Station (GTS). In the runup to the 100 millionth trade and beyond, Pokemon Company International will be preparing a special Fancy Pattern Vivillon on GTS. A special distribution event will occur in the future to distribute the new pattern once the milestone of 100 million trades on GTS has been achieved. An annoucement on the distribution will occur at that time.
For the uninformed, Vivillon has different patterns and colors depending on the location where it is caught in the real world. For example, Vivillons caught in Europe, Asia, and the United States could all have different patterns, though this is most likely done with the in game region selection, since the 3DS/2DS lack geolocation hardware.
Keep on trading those Pokemon and look for the special distribution event soon!