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Whilst being logged as a Jira's HR group user, from Navigation Bar Menu go to Management (1) ➜ Workweek Schemes (2). Now you can access workdays scheme's name to view it, or click "Edit" hyperlink directly. Currently viewed/edited workweek scheme is displayed below "Workweek schemes" (3). Whilst being in read only mode, you can quickly switch to edition by pressing "Edit scheme" (4) button.
You can choose what days of week will be marked as workdays. Every unticked day will be treated as a bank holiday and thus not taken from user's total e.g. yearly annual leave quota.
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