How is promotion forecasting different from normal demand forecasting?

Normal demand forecasting assumes the future will look reasonably like a continuation of recent history, adjusted for seasonality. A promotion breaks that assumption on purpose: it is a deliberate, time-boxed intervention designed to lift volume, and it also typically pulls some future sales forward and can distort demand for related products. Promotion forecasting treats the promotional event itself as an explicit input, modeling the expected uplift, the pull-forward effect on the following weeks, and the return to a normal baseline, rather than treating the resulting spike as noise to be explained after the fact.