This is the most obvious category. If you sell swimwear, your seasonality is dictated by weather. If you sell gifts, it’s dictated by holidays. However, look for secondary seasonality . For example, swimwear might peak in May for summer wear, but did you know there is a smaller spike in January for "winter escape" vacations?
Since you have no historical data, you must create a living seasonality calendar. Here is how to structure it for a new account: how to determine seasonality in a new sem account
Google Ads offers a "Seasonality Adjustment" feature in the bid strategies portfolio. However, do not use this for a new account immediately. You need at least a week of stable data for the algorithm to calibrate. For the first month, stick to manual CPC or Enhanced CPC to maintain control while you gather data. This is the most obvious category
If the client is in retail, you know Black Friday exists. If they are in B2B SaaS, you know December is dead (budgets are spent) and January is explosive (new budgets). However, look for secondary seasonality