To ignore seasonality requires assuming it has roughly the same effect on both groups. If they move in different directions or by very different amounts, you ca

To ignore seasonality requires assuming it has roughly the same effect on both groups. If they move in different directions or by very different amounts, you can't actually ignore it. Simple example: you're comparing your standard site's theme with a new pumpkin orange theme. In October, the pumpkin orange theme might go great, whereas in December, it might perform worse. There's clearly a seasonality interaction you need to account for. What you're thinking of is more like testing e-commerce button colors right after Christmas. After Christmas, it's likely that all your groups perform worse, but equally so.