Hiring is definitely seasonal, and there are probably more seasons than any of us in here can count.
Tax preparations companies in the U.S. start training seasonal tax preparers in the fall so they are ready to start working from January through April. Retailers in the U.S. start hiring seasonal workers in the fall for the Winter Holiday Season. Years ago, I worked at UPS as a seasonal worker loading trucks starting in early September. At that time, they brought on seasonal staff in late August or early September, and if you worked out through the Winter Holiday Season, then you might get offered full time employment. I got sick and was out in December, so I didn't get a full time gig.
There is a phenomenon known as "fishing for candidates", which some companies regularly engage in, particularly in the software arena. I've been in IT for 26 years, and I've had recruiters tell me about the phenomenon. I ask the recruiter about Position X with Company Y in City Z that I saw advertised, and the recruiter's response is, "Yeah, so?" Something of an odd response from someone who would make money if they presented my resume and I were hired. The recruiter would then give me a brief overview, from there perspective, about the position, what the company wants, and what the company is willing to pay. It doesn't sound very enticing, so I ask, "Is the company just fishing?", and the recruiter says, "Yes." They've learned the companies who engage in it, and perhaps have even figured out how the text of a job posting might somehow indicate they are just fishing and have no intention of actually hiring.
Some might reasonably ask the question, "Why would a company elicit for resumes if they have no intention of hiring? That wastes time and money." It consumes time and money, but it is not necessarily a waste. Fishing can be an easy way of finding out quickly what the available talent pool looks like for a given position. If the company got 2 resumes in 2 weeks, then this may be an area where the pickings are slim and the hiring process will take a longer period of time. If the company got 30 resumes in the first hour, then the talent pool is much deeper and they can afford to be very choosy. Posting a position and counting the resumes gives the company some useful data upon which to decide what is their next step.