Birdsong actually isn't a good example, because birds sing less the more noise there is. If there is more activity during certain days of the week, it will be b

Birdsong actually isn't a good example, because birds sing less the more noise there is. If there is more activity during certain days of the week, it will be biased. You really can't predict these kind of correlations ahead of time, which is why it is always critical that your data is not collected in a biased way. The data scientists should have randomly subsampled from each day of the week. This is extremely common practice, and there's nothing wrong with it if the subsampled data gives acceptably credible results. I've never heard of people only using certain days of the week. As a practicing and productive data scientist, that sounds completely insane to me. I would never hire a data scientist who does not understand why such practices are bad. Data scientist is just a buzzy name for statistician. Data scientists who do not understand basic good statistical techniques are not competent.