What is "Yukyu"（有給）?
"Yukyu"（有給）in Japanese is a 100% paid day off.
Even if you take a day off, your salary will NOT be reduced on that day. In Japan, employees are entitled to take "Yukyu" （有給）under the Labor Standards Law.
Paid day off: If they have been working for more than half a year since they were hired and have worked more than 80% of all working days, the number of days granted is 10 days.
Especially in recent years, Japan legally requires employees to take at least five days of paid leave per year. Employers will be punished if employees do not take leave. Employers will also want their employees to spend their vacation.
Basically you need to give notice to supervisor
It is your right to take a day off but you need to give notice and get approval from your supervisor beforehand.
However, applying for a vacation is unlikely to be rejected.
In other words, the rules for granting vacations do not allow your boss to refuse to take vacations without reasonable reason.
To reject your application, interfere with the normal operation of the business" is needed.
The factors that are judged to "interfere with the normal operation of the business" are as follows.
-Business scale and business content
-Job description and job characteristics of employees who applied for paid leave
-Difficulty in allocating alternative personnel, number of employees who specified paid leave in the same season
In short, it is quite rare case to reject your application for vacation. Do not worry and take a vacation!