Should seasonal swings in my revenue be explained upfront or left for the buyer to notice?

Explained upfront, generally — seasonal revenue patterns are extremely common in many industries, and buyers and their advisors are used to seeing them, but only when they're presented clearly as a normal seasonal pattern rather than left for the buyer to piece together on their own from raw monthly numbers.

The nuance is that seasonality itself is rarely the concern; what concerns buyers is anything that looks like it could be something else disguised as seasonality, such as a genuine decline that happens to coincide with your typically slow months, or cash flow timing issues that get blamed on seasonality without a clear pattern across multiple years to support it. Providing a few years of monthly or quarterly figures, with the seasonal pattern clearly labelled and explained, tends to reassure buyers rather than raise questions.

Waiting for a buyer to notice seasonal swings on their own, rather than explaining them proactively, tends to look worse than the pattern itself, since it can suggest you were hoping it wouldn't come up. A business lawyer and accountant can help you present multi-year financials in a way that makes a normal seasonal pattern easy for a buyer to recognize as exactly that.