If you're running an experiment over Christmas, people don't tend to behave the same during Christmas as they do the rest of the year. They definitely spend their money differently during that season, they're spending more time with their families at home, and they might be a little bit, kind of checked out of work, so people have a different frame of mind.

It might even be involved with the weather, during the summer people behave differently because it's hot out they're feeling kind of lazy, they're on vacation more often. Maybe if you happen to do your experiment during the time of a terrible storm in a highly populated area that could skew your results as well.

Again, just be cognizant of potential seasonal effects, holidays are a big one to be aware of, and always take your experience with a grain of salt if they're run during a period of time that's known to have seasonality.

You can determine this quantitatively by actually looking at the metric you're trying to measure as a success metric, be it, whatever you're calling your conversion metric, and look at its behavior over the same time period last year. Are there seasonal fluctuations that you see every year? And if so, you want to try to avoid running your experiment during one of those peaks or valleys.