What are the title and author of this (short) novel? It was for sale as a second-hand pocket-size book, mid-1990's; the author is almost certainly native to the English-speaking world. I have not been able to find a similar plot ever since. IIRC, the paperback contained only this one story, was rather thin with a black cover & cover art only under the title: the artist's impression of an alien factory.
I only read the first 2 chapters or so, most probably in English although it may conceivably have been a translation into French:
For several years, the Earth has been at war with an alien race and things go bad for us: they have built many bridgeheads all over the planet, like huge, imprenable citadels-factories. On the same day each year, at an otherwise unpredictable moment, the aliens subject the whole human race to hellish torment for no obvious reason, making us writhe and yell in horrible pain during an hour or so, the "hour of agony" or "hour of Hell let loose", I can't recall the locution.
Rising up again after the latest "hour of agony", a male protagonist observes that it does not start randomly as generally believed, but exactly 6hrs later or 18hrs earlier than on the previous anniversary day. From this he infers its triggering is astronomically determined: when the Earth is at a very special point on its orbit, the aliens try to open a wormhole & pull our planet en masse through, to their home stellar system.
The agony we feel is the burn of some radiation from their star seeping through the wormhole during the botched attempt. Should the next attempt succeed, the Earth will orbit this hellish star for good and the torture will be thousandfold and permanent.
So, the mysterious alien citadels are just the machinery needed to open the stargate. With this insight, the protagonists set out as a group, to try & destroy them before the next "hour of agony".
Added after LAK's answer. It is certainly not John Brunner's Age of Miracles nor his Day of the Star Cities, the accepted answer to sci-fi-novel-aliens-ignore-humans-put-up-giant-structures-all-over-earth. Extremely likely, Daniel F. Galouye's City of Force, the upvoted alternative to the same question, rather than LAK's primary answer Lords of the Psychon. I will change the tag from "novel" to "novella".
@LAK & all: which of the 2 stories includes a description of a "Horror Day" in their first pages? Can you excerpt the dialog which follows, expiaining each begins 6hrs later than the previous one? That would help.
Added after accepting LAK's answer I must have read a translation, after all: here's the cover I remember and the cover artist seems to have only worked for Denoël, not for US publishers. The title translates back to Lords of the Spheres, so obviously "Lords of the Psychon".
