...
Whilst being logged as an Jira's HR group user, from Navigation Bar Menu go to Management (1) ➜ Holiday Schemes (2). Now you can access scheme's name to edit it, (3) or click "Edit" hyperlink (3) - both actions have the same effect.
Edition button for a holiday scheme is also visible after accessing/editing a desired scheme. If you do not wish to go with creation of new holiday schemes and assign particular users to it, then fast track mode is enough for you (i.e. working only on Default holiday scheme), then you can use "Add holiday" button (5) to add off-work day to the calendar.
...
To remove holiday, simply click its name on calendar and in "Edit holiday" prompt click the "Remove" button.
Workdays
Analogically to the Holidays, access to Workdays is quite similar:
%20-%20workweek%20schemes.png?version=1&modificationDate=1519047182682&cacheVersion=1&api=v2)
Whilst being logged as an 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 swich to edition by pressing "Edit scheme" (4) button.