How do I set holidays and weekly offs?

In Settings → Attendance rules (admins and HR):

• Holidays: add each holiday with its date, for everyone or one city. Masthead doesn't add public holidays for you, so add your year's list.
• Weekly offs: the days each person is off. A person's work week (five or six days) sets them to start with.
• Shifts: your working hours, and the hours that make a full day.