As you can see, you cannot create the events “US Holidays“. The events are coming by iCal feed. The same if you are using the option URL, during iCalendar source configuration.
Creating worklog events based on iCalendar events
Did you know that you can create a worklog event based on an event imported from your Google calendar? To do this, simply click on iCalendar event. Then, click Create worklog button, on the information dialog for the event.
Note that in case the iCalendar event contained an issue key in its summary or description, the issue key will be automatically populated to the Issue field on Create Event dialog, assuming the issue complies with the worklog source configuration requirements.