This study proposes a framework for classifying the causal effects of tourism demand seasonality, linking between season effects (interseason) with within season effects (intraseason). The literature has typically investigated the causes of tourism demand seasonality as isolated static factors, mostly ignoring the possibility of interseason and intraseason impacts. This study contributes to the literature by providing a system of categorizing the drivers of tourism demand seasonality, by introducing additional causal factors, and by using an alternative seasonal smoothing technique. The methodology includes data decomposition, panel unit root tests, and instrumental variable regression, distinguishing between peak and shoulder seasons. The findings, based on a case study, identify vigorous–irregular and moderate–irregular factors being the most prominent drivers of U.S. tourism demand seasonality, which could assist in specific policy recommendations for more sustainable management of tourism demand seasonality.