We Analyzed 5,000 Google Ads Accounts: Seasonality Adjustments Backfire on Black Friday

A detailed analysis reveals that applying seasonality adjustments during Black Friday–Cyber Monday leads to higher costs and lower returns on ad spend compared to letting Smart Bidding operate without interference.

AI · Large-scale, multiyear account analysis suggests manual seasonality adjustments can undermine automated bidding, informing how advertisers configure algorithmic optimization.

What three years of BFCM data shows

We measured each BFCM period from the Wednesday before Black Friday through the Wednesday after Cyber Monday. Then we compared it with a pre-holiday baseline (from two Tuesdays before Black Friday to the Tuesday before).

Advertisers were split into two groups: those who set seasonality adjustments for the period and those who didn’t. Together, the three years cover tens of billions of impressions.

Three things stand out:

CPCs consistently rose about twice as fast with adjustments. Every year, telling Google to expect higher conversion rates translated directly into higher bids. Advertisers who didn’t intervene still paid more for Black Friday clicks, but their costs rose only half as much.
The extra revenue is getting more expensive. Adjustments did buy volume. But the gap is closing. In 2022, advertisers using adjustments grew conversion value about 25 points more than everyone else. By 2024, the advantage had shrunk to about 6 points, and it cost them a ROAS swing of more than 21 points. That’s a lot of efficiency to give up for a small amount of extra revenue.
The Black Friday conversion rate bump is shrinking. Among advertisers who didn’t use adjustments, conversion rates rose 17.5% over baseline in 2022, 11.9% in 2023, and just 7.5% in 2024. Shoppers now spread their purchases across a longer promotional season, flattening the peak compared with before. As a result, anyone who sets an adjustment based on what Black Friday “usually” does is likely overestimating the lift.

Why seasonality adjustments backfire on Black Friday

A seasonality adjustment tells Smart Bidding: “Conversion rates are about to rise by X%. Bid accordingly.” The system takes you at your word and raises bids in proportion.

That creates two problems on Black Friday:

Google already knows Black Friday is coming. Smart Bidding is already trained on years of BFCM behavior across millions of advertisers. It also reacts to real-time auction signals as the week unfolds. Consequently, when you add an adjustment on top, you’re asking the system to anticipate a lift it’s already expecting. This leads to the expectation being counted twice.
You have to predict the lift precisely, and almost nobody can. If you guess conversion rates will rise 20% and they rise 7.5%, as the average did in 2024, you’ve told Google to overbid by a wide margin. And it will do that across every auction at the year’s most expensive point, before you’ve had time to notice.

Seasonality adjustments still have a job. They’re built for events Google can’t see coming:

Your own flash sale or surprise clearance event.
A one-off “friends and family” promotion.
A big email or SMS push to your customer list.
A niche industry event with no broad historical pattern.

For Black Friday, Cyber Monday, Christmas, and other widely recognized holidays, the data says to leave the adjustment off.