Seasonality is the pattern of predictable ups and downs in demand across the year. Ice cream sells in summer, tax software in spring, gifts in December. The pattern repeats, so you can plan for it: stock, staff, cash and marketing budget follow the curve instead of the calendar month average.
Seasonality decides when you need cash, not only how much. An example: your shop makes 240,000 euros a year, and 96,000 euros of that, 40 percent, arrives in November and December. The other ten months share 144,000 euros, or 14,400 euros per month. If your fixed costs are 15,000 euros a month, you lose 600 euros in each of those ten months, 6,000 euros in total, and earn it all back plus 66,000 euros in the two strong months. The business is profitable and can still run out of money in September if you did not plan for it.
The practical consequences: order stock months before the peak, agree payment terms with suppliers that match your inflow, and size your cash buffer for the longest weak stretch, not for an average month.
Two errors show up often. The first is extrapolating a single month. December multiplied by twelve is not an annual forecast, and neither is August multiplied by twelve. Use at least twelve months of data, or an industry pattern if you have none of your own. The second is mixing up seasonality with growth. If sales rise from March to June every year, that is the season, not traction. To see the real trend, compare each month with the same month last year rather than with the month before.
Seasonality also affects hiring and advertising. Bidding on the same keywords in December, when competitors bid too, costs more per click than in February. Sometimes the cheaper move is to build demand before the peak starts.
