I suggest to show the Set/Delete holiday menu to admins on the global page
If you select Work time from the top right menu (that is, not linked to any project), the button "set holiday on this date" is not displayed.
I suggest changing app/views/work_time/show.html.erb line 10 from
<%if @project && User.current.allowed_to?(:edit_work_time_total, @project) then%>
<%if ((User.current.admin) || (@project && User.current.allowed_to?(:edit_work_time_total, @project))) then%>
so that global admins still get the right to set bank holidays for the all teams without selecting a specific project. This is all the more justified that the holiday table does not store any associated project.