They’re both busy.
As others have mentioned, one of the main issues with Monday is that the perception is that they’ll be light, and then they’re usually not - also, Mondays are usually shorter hours, less staffing, less entertainment offerings, etc. compared to the weekends. So even if a Monday actually has less people than a Saturday, it can “feel” more crowded, especially to people who were expecting light crowds (which, honestly, no one should ever expect at this point)...
We go on a fair number of Mondays (we are headed there after school today), and they’re not as good as Wednesday/Thursday (usually the best days IMO), but the crowds are usually not terrible. They’re just unexpected for a lot of people.