The premise is that Earth was invaded by a race of giant hamster-like aliens called the Ruhar. They struck hard, fast, and efficiently against Earth's infrastructure with seemingly no provocation. The Kristang, a race of lizard people, promptly recruit humanity's armies to join in the interstellar war against the Ruhar. There's just one problem: the Kristang are the bad guys. Well, two problems, the Kristang are the bad guys and humans are so woefully outmatched that they have absolutely no chance of fighting back if any of the other galactic powers decide to look at us funny.
It seemed that the Kristang are a conquering evil bunch of lizards and the Ruhar attacked Earth because they knew once humanity was in their sights that we would soon be their slave state. So, they decided to blow up our infrastructure before we could end up using it to help the lizard men. Which was something of a dick move on their part. However, this is a galaxy where even the nicest races are not terribly sympathetic to a bunch of backwater primitives from an obscure body in the Sol system.
Joe Bishop joined up to fight the "evil" Ruhar and defend his planet but is rapidly made aware of just how awful his allies are. The Kristang practice eugenics, cultural misogyny, and regularly commit war crimes that would make the Klingons or Galactic Empire blush. He soon finds himself in hot water after initially impressing them with his hamster slaying skills. He finds himself promoted to the status of Colonel despite the US military (or any military) not working that way and soon finds himself way in over his head but also the authority to do something about it.
I'll spare you more spoilers, but everything changes thanks to Joe's discovery of an AI manufactured by the mysterious Elder race. The adorably named Skippy the Magnificent is no Cortana, though, as he's a crass and annoying know-it-all who can barely be convinced to help a monkey like Joe. However, he's been sitting on a shelf for millions of years so it's not like he has anything else better to do.
Columbus Day is a work that has its flaws. Craig Alanson tends to over describe things and use "tell rather than show" a lot but as a first-time novel, I was still engrossed with his work. Joe Bishop and Skippy's developing relationship is something I really enjoyed with the characters having ample room to refine their comic relationship. It is also a well-developed universe with the premise of humanity being utter babes in the woods being something I appreciated.
The book is at its most enjoyable when it embraces its inherent silliness and the sheer insanity of Joe's relationship with Skippy. The early parts of the book and worldbuilding are a bit hard to get through but I really enjoyed it once we got to the fact Joe is walking around with a weapon more powerful than the Death Star yet possessed of a childish elitist personality. Basically, it's as if you combined Halo with The Stanley Parable.
I should note this is one of those works that is better listened to rather than read. In his Afterword, Craig Alanson actually admits this himself as he says that RC Bray's narration is what turned his novels from a quirky military science fiction series into an international success. I strongly recommend the audiobook version and think it's well worth getting the entire series (though that may take a while to listen to).