The onset of acute myocardial infarction (AMI) has been shown to occur in a reproducible pattern with a peak in mid-morning and a secondary peak in late afternoon and early evening. More detailed information on the timing of this catastrophic event may provide important pathophysiologic information. Using the database from the Holter Registry of the Cardiac Arrhythmia Suppression Trial (CAST) (n = 22,516), the day of the week, the month, and season of the onset of AMI was obtained and correlated with demographic characteristics. The pattern of the day of onset for the entire population was significantly nonuniform (p <0.0001) with a Monday peak and a weekend nadir. This pattern was observed in most of the examined subgroups. Analysis of seasonal data revealed nonuniform distribution (p <0.001) with a peak in winter and autumn. We conclude that AMI is not a random event but occurs in definite patterns related to the day of the week and the season of the year. These patterns were observed in a wide variety of patient subgroups and appear related to climate, occupation, and other factors.