What is seasonality?
Seasonality is the presence of regular and predictable change in time series data at specific intervals within a year-long window. These changes may occur monthly, weekly, daily, etc. Factors that may affect seasonality include the day of the week, the month of the quarter, the time of day, or any other recurring time interval.
Seasonality differs from cyclical patterns because the latter don’t occur at regular intervals within the context of a year-long window. Seasonality differs from stationarity because the latter is not predictable.
Seasonality is useful in forecasting models, especially for demand forecasting.