Seasonality adjustments is available from 3 September 2019 for Search and Display campaigns, but not for Video or App campaigns. It should be coming to Shopping campaigns later on the year.
Google says: With Seasonality adjustments, you can apply a predicted conversion rate adjustment and Smart Bidding will consider that adjustment for the date range selected while trying to hit your target CPA. https://support.google.com/google-ads/answer/9461281
Effectively, you are applying some manual input into Smart Bidding to predict your conversion rate during your promotion.
The system adjusts according to your estimated conversion rate, and then it reverts back to before seasonality adjustments.
TOOLS & SETTINGS > Bid Strategies > Advanced Controls > Seasonality adjustments > + New seasonality adjustments.
Select start/end dates and times > Select the campaign type (Search or Display) or a Specific campaign > Select the Devices (Computers, Mobile Phones or Tablets) > Select your Conversion rate adjustment.
Enter an estimate based on expected conversion rate changes. For example, if you expect conversion rates to increase by 50% during a 3-day sale, add up to a +50% conversion rate adjustment. This adjustment helps optimise your bids.
Watch it on YouTube (Larger version)
Extract from my Book ‘Making Google Ads Work’.