Seasonal sales forecast from last year's data: stock and ad budget for December

.comparison-cards { display: flex; flex-direction: column; gap: 1.5rem; } .comparison-card { background: #fff; border-radius: 16px; padding: 20px; border: 2px solid #efefef; } .comparison-header { display: flex; justify-content: space-between; font-size: 16px; margin-bottom: 12px; } .comparison-row { display: flex; justify-content: space-between; padding: 8px 0; border-top: 1px solid #efefef; font-size: 14px; color: #1f2937; } .comparison-label { font-size: 14px; width: 30%; } .comparison-value { width: 65%; text-align: right; } @media (max-width: 600px) { .comparison-row { flex-direction: column; align-items: flex-start; } .comparison-value { text-align: left; margin-top: 4px; } } In November almost every online store owner asks me the same two questions: how much stock should I order for December, and how much should I put into ads? A good answer starts with your own numbers from last year, not with gut feeling and not with what competitors seem to be doing. Below I show the method I use: align the weeks, clean the data, apply the trend and the effect of promotions, then turn the forecast into a stock order and an ad budget. Black Friday falls on Friday 27 November 2026 this year and Christmas Day is also a Friday, 25 December, so time is short. At the end there's a calculator that does the whole thing over 8 weeks. 1. Last year is the starting point, not the answer Last season's sales show you the shape of the season in your store: when people start buying, how big the Black Friday peak is, how long gift shopping lasts and when everything stops because of delivery cut-offs. That shape is usually more stable than the volume. Volume changes with your ad budget, your prices, new products and your customer base. So I work with a simple formula you can check at any time: the forecast for a week is the sales from the matching week last year, after cleaning, multiplied by the growth trend and, where relevant, by the effect of a new promotion. A calculation you…