Most seasonal slowdowns aren't a surprise, they show up in the data a year in advance. The problem is usually that nobody looked.
The exercise
Pull two to three years of monthly revenue. Slow months tend to repeat. Once you can see the pattern, you can plan cash flow, inventory, and marketing spend around it instead of scrambling when it hits.
What to do with the lull
Use predictable slow months for retention campaigns, content production, and system-building work, the things that are hard to prioritize when the business is at full sprint.