Anyone who got admitted to a Swedish hospital with symptoms resembling an acute heart problem between 1998 and 2013 was automatically enrolled in the SWEDEHEART study. Researchers gathered detailed data about them: What time did their symptoms start? What was the eventual diagnosis? That way, cardiologists could figure out whether more people are having heart attacks around Christmas or if heart attacks are more deadly at that time of year. These cardiologists decided to also look at other holidays and events of national import. Christmas and New Year’s are the main festivities, but there’s also Easter, FIFA World Cup, and Sweden's Midsummer holiday.