Capturing black Friday effect in seasonal adjustment with an application to the Swedish retail sales

Black Friday’s rapid growth in Sweden since its introduction in 2013 poses challenges for the seasonal adjustment of Swedish retail sales index. Long-term seasonal effect patterns fail to reflect the short-term presence of Black Friday, which could cause misleading seasonally adjusted results for retail sales and household consumption, in particular for November and December months. We demonstrate this problem with the increasing seasonal-irregular ratios from seasonal adjustment. We propose to use explanatory variables for capturing and estimating Black Friday effect and three explanatory variables are suggested, with gradually increasing magnitude for November months. Seasonal adjustments using those explanatory variables included in the RegARIMA model showed significantly improved results. The explanatory variable in an exponential form has extra appealing properties for our purposes.