Physical commodities are produced and consumed on a calendar. Corn is harvested in autumn and stocks are tightest just before the next crop; natural gas is injected into storage from April to October and withdrawn from November to March; gasoline demand peaks in the summer driving season.
Seasonality shows up in the forward-curve as much as in flat price. A contract for the month after harvest is structurally cheaper than the one before it, and the shape repeats every year, which is why intramarket-spread traders study seasonal charts rather than price charts.
The caution is that seasonality is a tendency, not a rule, and it is already partly in the curve. Trading a pattern that shows up in eight of the last ten years still leaves the two years that ruin an account, and the sample-size behind most seasonal statistics is tiny.
Example: natural gas typically bottoms in September as injection season ends. Buying October gas every year from 2010 to 2020 would have won most years, then lost heavily in a warm winter when the expected rally never came.
Related: forward-curve, intramarket-spread, natural-gas-futures, crop-year, sample-size