The Japanese financial year ends on March 31st and the new one starts the day after (odd, huh?), and the school year follows the same pattern with the new school year starting in the beginning of April. So from this perspective the Japanese system works quite differently from the West where this is coordinated with the end of the year in December and then the second semester starting six months later.
What happens is that the larger Japanese companies have huge hiring events for the new university graduates and hire them en-masse at the same time with huge ceremonies and plenty of training. Now, Japanese larger traditional companies do not necessarily hire new graduates for specific positions since it is widely known that Japanese kids don't learn anything at the university at all, unless they happen to have a Masters or Ph.D. degree. So the companies take care of the training and sort the kids for jobs as they see fit after tests, training etc.
During this time of year they are everywhere and are very easily recognized due to the cheap looking suits; dark suits for both the boys and the girls with white shirts/blouses and discreet neckties for the boys. Sometimes they are seen with some older employee herding them around and they look half-scared to death and focused on behaving like responsible adults. At other times they can be encountered without their keepers, on their home from some company arranged event where they let their hair down and behave a bit more rowdy; reminding me more of high school kids in awkward looking suits than anything else.
This will calm down very soon as they get more settled down in their companies, have to take the manner classes to learn how to answer the phone properly and behave like regular people (ok, Japanese business manners do take some time to learn properly, even for most Japanese kids with university degrees).
The hordes of new employees doesn't really annoy me per se, since it's over pretty quickly and they face into the companies that hired them, but it can be a bit annoying since it can be hard to just drop into a nice looking izakaya since they're fully booked up with welcome parties for the new hires.
Seeing them awkwardly standing around outside an office, waiting for their Shepperd reminds me of baby penguins standing around waiting for their masters to bring them scraps of food...
My little company has gone around this whole issue and just hire new people with previous experience, saving us the trouble of having year long training programs inside the company and getting people who actually know how to behave themselves!