April is the start of the fiscal year in Japan, and as it also comes immediately after March graduation ceremonies, April is also when most new graduates being their first jobs after finishing college. During their final year of studies, most students are also researching companies and going to interviews. At the start of April, many large companies will hold entrance ceremonies for the many new employees who will be starting.

https://piece-of-japan.com/culture/annual-event/nyushasiki.html