Seasonality has become a key problem facing the tourist sector, constituting a major threat to sustainable growth, destination image, and loyalty, especially in regard to large-scale, well-established destinations. A dynamic model, combining natural and non-natural explanatory variables, is used for a provincial panel data set over the 2006-2015 period to identify determinants of seasonality for Spain's main markets of origin. We show that the inertia factor, economic variables, and climatic variables are significant. Some of the most interesting results are the identification of common patterns across the main markets of origin. Considering, for example, high temperatures in the low season, our results show that, if they occur in the destination market, they reduce seasonality; in contrast, if they occur in the markets of origin, they increase seasonality.