Across 5,886 Google Ads accounts during BFCM 2024, advertisers who applied seasonality adjustments saw CPCs rise 34%. Those who didn't? CPCs rose 17%. The adjustment group's ROAS fell 15.7% while the hands-off group's ROAS climbed 5.7%. That's a 21-point ROAS swing for the privilege of telling Smart Bidding something it already knew.
Optmyzr ran this comparison across three consecutive BFCM periods (4,292 accounts in 2022, 5,114 in 2023, 5,886 in 2024), and the pattern held every year. The data, reported by MarTech, is observational, not a randomized experiment. Advertisers who use seasonality adjustments may also run more aggressive volume strategies. But three years of the same directional result, at this scale, is hard to dismiss.
The Double-Counting Problem
A seasonality adjustment tells Smart Bidding: "Conversion rates are about to rise X%. Bid accordingly." On a flash sale Google can't anticipate, that's the right tool. On Black Friday, it creates a specific failure mode.
Smart Bidding already trains on years of BFCM behavior across millions of advertisers. It also reads real-time auction signals as the week unfolds. When you layer a manual adjustment on top, the expected conversion-rate increase gets counted twice, and the result is overbidding at the most expensive auction window of the year.
The second problem is precision. Google's own documentation says seasonality adjustments require you to estimate the percentage change in conversion rate. If you guess 20% and the actual lift is 7.5% (the average for non-adjustment advertisers in 2024, per Search Engine Journal's coverage of Optmyzr's study), you've told Google to overbid by a wide margin across every auction before you've had time to notice.
The Shrinking Peak Makes It Worse
Among advertisers who didn't use adjustments, conversion rates rose 17.5% over baseline in 2022, 11.9% in 2023, and 7.5% in 2024. The BFCM conversion-rate bump is compressing year over year as promotional windows stretch longer. Anyone setting an adjustment based on what Black Friday "usually" does is calibrating to a peak that no longer exists.
This matters even if you're running B2B SaaS campaigns with nothing to do with retail. BFCM distorts auction dynamics across verticals. If your lead gen campaigns are running seasonality adjustments during that window, you're paying inflated bids against an overestimated conversion-rate signal, and your offline conversion feedback loops (which typically lag days or weeks) can't correct fast enough.
When Seasonality Adjustments Still Earn Their Keep
Google's documentation is clear on the intended use case: short events (1-7 days) where Smart Bidding is unlikely to anticipate the conversion-rate change. Your own flash sale, a one-off promotion, a niche industry event with no broad historical pattern, or a big email/SMS push to your customer list. These are signals the algorithm genuinely can't see coming.
For widely recognized holidays (BFCM, Christmas, Prime Day), the data says the safer default is to leave the adjustment off. A blanket "never use" stance would be too strong; an account with strong historical evidence of a predictable, material CVR spike that differs from typical patterns might still benefit. But the burden of proof should sit with the adjustment, not with the default.
What to Do Instead: Change the Target, Not the Forecast
If the business wants to trade efficiency for volume during BFCM, there's a more direct lever: lower your tROAS or tCPA target for the event window. This tells Smart Bidding what you're willing to pay for incremental conversions, without asking it to predict a conversion-rate change on top of the one it's already modeling.
- Audit for scheduled adjustments. Check every account for seasonality adjustments set between late November and early December. In agency environments, a client or teammate may have added one "just in case." Google allows adjustments across up to 2,000 campaigns, so a single misconfiguration can ripple account-wide.
- Verify conversion signal hygiene. Compare Google Ads conversion data against GA4 for the last 30 days and flag accounts with more than one primary purchase or lead conversion action. Double-counted conversions trick Smart Bidding into overbidding, mimicking the exact problem adjustments cause.
- Set failure-mode alerts before the sale starts. Define CPC anomaly thresholds, budget pacing monitors at portfolio level, and hard bid caps where you need a ceiling. Decide what "too expensive" looks like before auction pressure makes the math fuzzy.
- Freeze structural changes. New campaigns, bid strategy swaps, and major target changes force Smart Bidding to relearn during the most expensive traffic window of the year.
The Governance Frame
Seasonality adjustments are labeled "advanced" in Google's interface for a reason. They're a manual override on an automated system, time-bounded and conversion-rate-specific. For most accounts during BFCM, the right move is to let the automation do the job it was trained for, and redirect your effort from forecasting the week to protecting it.
Three years of data across more than 15,000 account-years point the same direction. The algorithm already priced in Black Friday. The adjustment just made you pay for it twice.
