The Holiday Sync App for Project Online and on-premises allows you to import holiday calendar exceptions into your Project Server base calendars, a task which in the past needed to be done for each calendar at least once manually for all future dates.
Holiday Sync imports calendar holiday data from a free online web service available on the web at www.holidaywebservice.com.
This app was developed for Project Conference 2014 to showcase JavaScript, read more about it here: http://nearbaseline.com/blog/2014/01/preview-extending-project-online-and-on-premises-with-javascript-apps/