Schedule for application is same for both Master's and PhD course.
Only Master's course applicant can choose enrollment in October or April, depends on personal situation such as visa, no matter when the applicant has taken the examination.
If you want to enter as research student and prepare entrance exam in Japan, the application period is early January for April enrollment and early July for October enrollment.
Applicants for Master's course need to prepare
Application form and resume (Curriculum vitae)
Graduation certificate (diploma)
Transcript from last school attended
Application essays
Score sheet of and English language proficiency examination (TOEIC, TOEFL, IELTS) ; Can be changed to written test by COVID-19
Recommendation letter from your academic advisor at the last school attended
Examination/application fee: 30,000 yen
Applicants for PhD course need to prepare
Application form and resume (Curriculum vitae)
Graduation certificate (diploma)
Application essays
Score sheet of and English language proficiency examination (TOEIC, TOEFL, IELTS) ; Can be changed to written test by COVID-19
Recommendation letter from your academic advisor at the last school attended
Master's thesis (3 copies)
Master's thesis abstract (3 copies) ; either Japanese (about 3,000 words) or English (1,200 words)
Examination/application fee: 30,000 yen
Applicants for Research student need to prepare
Application form and resume (Curriculum vitae)
Graduation certificate (diploma)
Transcript from last school attended
Examination/application fee: 9,800 yen
I recommend you to contact tightly with your supervisor not to miss any of schedule or documents.
After you pass the entrance exam, you will receive a Letter of Acceptance from Hokkaido University, that would be required for studentvisa application.