What they are for
When you know conversion rate will jump for a day or two, a promo or a product launch, a seasonality adjustment lets Smart Bidding anticipate it instead of learning after the fact. They work best for events of 1 to 7 days, and Google warns they may not work well beyond 14 days; Google already handles regular seasonality on its own.
Not the same as data exclusions
Do not confuse them with data exclusions, which tell Google to ignore a past period when tracking broke or conversions were distorted. Seasonality adjustments look forward at expected conversion-rate changes; data exclusions look back at bad data. Overusing either, or using them for long trends, does more harm than good.
Common questions
When should I use a seasonality adjustment?
For short, predictable conversion-rate spikes or dips of about 1 to 7 days, such as a flash sale. For normal seasonal patterns, Smart Bidding already adjusts on its own.
What is the difference between seasonality adjustments and data exclusions?
Seasonality adjustments prepare bidding for an expected future change in conversion rate. Data exclusions tell Google to disregard a past period, usually because tracking was broken.