work day; day of attendance
Refers to a day an employee is scheduled to work or actually goes to work, often contrasted with days off.
Tomorrow is a work day, so I'll go to bed early.
There are 20 work days this month.
出勤 is the act of going to work, while 出勤日 specifies the day itself.
Compound of 出勤 (attendance at work) and 日 (day).