My father had a heart attack on Monday, so I believe this. Anyone who has to drive for at least an hour in heavy traffic will understand why.
Mondays are just stressful. We have so many things on our minds. We make to-do lists for the rest of the week, we stress about that new project, or the new diet that we couldn't follow through with. It's the first day of the week and people feel more responsibility and more tension on this day.
As for why Saturday is the second day when most heart attacks occur, I wonder if that has to do with binge drinking. Most people wait till the weekend to drink and party. Alcohol can definitely bring on a heart attack in someone with poor cardiac health.