In the post-apocalyptic manga (and anime) Kaze no Tani no Nausicaa by Hayao Miyazaki, "Seven Days of Fire" is the name given to the war in which the earlier industrialist civilization (our civilization) tore itself apart.

The name of one episode of Patlabor, "The SV2's Seven Days of Fire", is an in-joke in referral to Nausicaa.

Here is the full prologue of the manga, which describes the situation:

In a few short centuries, industrial civilization had spread from the western fringes of eurasia to sprawl across the face of the planet. Plundering the soil of its riches, fouling the air, and remolding lifeforms at will, this gargantuan industrial society had already peaked a thousand years after its foundation: ahead lay abrupt and violent decline. The cities burned, welling up as clouds of poison in the war remembered as The Seven Days of Fire. The complex and sophistocated technological superstructure was lost; almost all the surface of the earth was transformed into a sterile wasteland. Industrial civilization was never rebuilt as mankind lived on through the long twilight years...

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