"San Lunes" does not correspond to "lunes de infarto".
That's used when someone doesn't go to work on Monday (because they're suffering from weekend's excesses, or they're just feeling lazy), is as if they were celebrating some holy day.
People having a "lunes (or any other day) de infarto", are having a bad day, at work or not, and usually because they're going through a very busy or a specially unpleasant day.
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