Holidays are not something that can be catched by STL decomposition. Instead you should use a model that enables you to include holiday dummies. One such model is [Facebook's prophet][1], that is able to decompose the time-series to seasonal patterns, trend, include specific dummies, and allow for structural changes by detecting changepoints and using them as variables for piecewise regression.


  [1]: https://research.fb.com/prophet-forecasting-at-scale/