Seasonality
Regular patterns tied to the calendar — weekdays, paydays, holidays, sales seasons — that move metrics regardless of what the team did.
Formulacompare like with like: same weekday, same month last year
How we work it through
- 1
Plot at least a year before judging a month.
- 2
Compare with the same weekday or the same month last year.
- 3
Mark holidays and promotions on the chart.
On the practice dataset
The query we run on Meridian, Atterna’s practice marketplace, and what it returns.
SELECT dayname(day) AS weekday, ROUND(AVG(parcels))
FROM meridian.daily_parcels
GROUP BY dayname(day), dayofweek(day)
ORDER BY dayofweek(day);ResultMonday 412 parcels a day, Saturday 280. December 2025: 413 a day, January: 275.
The common trap
Comparing a week with a holiday to one without, or this December with last month instead of last December.
When an AI answers
An AI assistant
“January volume fell 33%, something is wrong.”
- What it didn’t know
- It compared January with December and did not know December is always the peak.
- Ask back
- “How does this January compare with last January, and with a normal month?”
Read next
Examples use Meridian, Atterna’s fictional practice marketplace.