Lunes de infarto

[poli]

Does this mean manic Monday?

[Tomisimo]

I've never actually heard that term, but your translation sounds right. I found a tweet, that from context, seems to confirm it:

Lunes de infarto, como siempre. Presentación de catalogo con éxito y esta tarde de imagen corporativa con un cliente bastante difícil.

[chileno]

Maybe more at hectic. I've never heard of manic used that way, so I am not sure, although I understand it.

[AngelicaDeAlquezar]

"De infarto" is something that greatly impresses/affects/surprises/upsets you... as for causing a heart-attack.

In the case of Monday, it might be an extremely busy day and/or a day full of unhappy events.

[JPablo]

I agree with the above. Oxford Superlex gives good examples:
infarto = heart attack
como se entere su madre le va a dar un infarto (familiar) = if his mother finds out, she’ll have a heart attack

de infarto (fam): fue un partido muy reñido con un final de infarto = it was a hard-fought game with a heart-stopping finish
una noticia de infarto = incredible o staggering news

In the sense of 'hectic'/'manic' (frenzied, agitated)/'extremely busy', this usage seems slang that has become part of the common language. I do not see it in DRAE nor Moliner Dictionary of Usage (this last one being pretty up-to-date with 'usage' definitions...) "Diccionario de argot" by Julia Sanmartín Sáenz (Espasa publishers) however, gives this applicable definition:
de infarto loc. Con prisas. Sorprendente. Se dice que los infartos vienen causados por sensaciones fuertes -prisas, sorpresas, etc.- y de ahí que este término se emplee por metonimia -el efecto por la causa- para denominar este tipo de estados emocionales. La noticia fue de infarto; nunca creí que pudiéramos tener trillizos.

En búsquedas de Google encuentras cosas como que "a tal y tal político le espera un día de infarto..." o "chicas de infarto [impresionates, por lo bonitas...]" "Hoy ha sido un dia de infarto de miocardio, de esos que no se olvidan…" (Casi 2.000 hits con la expresión... así que si queréis ejemplos, 'haylos') Quizá este smilie sea lo más cercano, y este sea la antítesis...

[CrOtALiTo]

Originally Posted by poli

Does this mean manic Monday?

It could mean an day less wanted.
As on Mexico is said San Lunes.
The people don't want the Monday day, because recently has gone the Sunday and no ones want to go to work on Monday.

[hermit]

Hey CrO - "...a day less wanted" works well, but better said maybe
"not everybody's favorite day" or something, but I like the clarity of
"a day less wanted"...sounds perfect like a dictionary definition...

¡Ha! San Lunes...

hermit

[CrOtALiTo]

Yes the Monday casually became in a San Lunes a day not very favourite for the people.

[AngelicaDeAlquezar]

"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.

[JPablo]

I agree with Angélica... but I had never heard "San Lunes", which is really funny to me... and I am surprised that something like this was not in use in Spain... (the cradle of 'siesta' tradition and other highly lazy activities...)