Fundamentals

Seasonality

Seasonality is a pattern that repeats on a fixed clock — weekly, yearly, intraday — because people, weather, and operations do. It is not a trend, and it is not a one-off spike.

Retail has a Wednesday lull and a December pile-up. Grids have an evening peak. Hospitals have a Monday clinic bulge. If your forecast cannot say which weekday it is, it will spend its life apologizing.

The period matters. Weekly seasonality wants a 7-day lag. Yearly seasonality wants more than one trip around the sun, or a borrowed shape from a related series. Multiple seasons can stack: hour-of-day sitting on day-of-week sitting on month.

Do not confuse a one-time event with a season. A viral tweet is not 'seasonality.' A promo you run every Black Friday is closer — if you will run it again, put it in the calendar, not in the residual.