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Background: A circadian rhythm of myocardial infarction has been described but there is little data on its relation with seasons and months. Methods: From June 2013 to June 2018, we analyzed the alerts for acute ST-segment elevation myocardial infarction (STEMI) in a Spanish region with 6.64 million inhabitants, universal health coverage, and an organized STEMI reperfusion network. We selected those patients which an identifiable culprit plaque. Results: We recruited 6765 cases of STEMI due to type I acute myocardial infarction (type-I AMI), with mean age of 63.2 years (range 17–101, standard deviation [SD] 13.7), 5238 were males (77.4%) and 2801 (41.9%) were 65 years or older. The hourly distribution followed a fixed pattern in all months, with most of the events occurring between 6:00 AM and 4:00 PM, a peak at approximately 01:00 PM and a valley between 10:00 PM and 06:00 AM. No significant difference was found when comparing the mean time to first medical contact between July (the month with more daylight hours) and December (the month with shortest days). No significant differences were found between male and female patients, or between patients aged 65 years or older and younger patients. There was a close correlation between the number of events per month and the number of events occurring during the day (6 AM to 6 PM, r = 0.988, p = 0.001) and during the night (6 PM to 6 AM, r = 0.944, p < 0.001), with different slopes of the regression lines (t-test, p < 0.001), so that the difference between day-night occurrences increased with the total incidence. Conclusions: There is a circadian pattern in the presentation of STEMI that is not influenced by sex and age. The different incidence of STEMI at different times of the year does not affect the circadian pattern in terms of the shape of the curve or the mean time of presentation, although diurnal events increase more than nocturnal events, suggesting that triggers are most likely to act during vulnerable periods as determined by a circadian-based rhythm.