Seasonality
Why do traders look for calendar patterns in prices, and why can a few years of data never prove one exists?
In plain words
Seasonality is the idea that a market tends to behave in a certain way at certain times of the calendar year, such as specific months or weeks repeating a pattern of gains or losses. It is a tendency drawn from past repetitions, not a rule built into the market.
Why it matters
Traders may size positions or expect a directional move because a week or month has historically behaved a certain way. The danger is sample size: a calendar pattern observed over a handful of years contains only a handful of independent observations, so one unusual week can create or erase the pattern entirely.
In a real week: BTC, week of 27 April 2026
The week of 27 April 2026 had 428 weeks of history to examine, roughly eight years, and BTC closed -0.11% for the week. The four prior weeks were all up (4.58%, 2.47%, 4.33%, 6.58%), a run that a seasonality claim might cite as evidence of a recurring spring pattern. Yet this week's return sat at the 46.0 percentile versus history, with volume at 0.61 of its prior 12-week average, offering nothing a calendar rule could reliably have predicted.
A common mistake
Treating a pattern seen over a few years as established. Each calendar period repeats only once a year, so the number of true trials is the number of years, not the number of days or weeks in the sample.