A holiday is a special day reserved to honor some notable figure or event. It can also to refer to a series of days. 
Patch 7.1.5 introduced a shorter version of these types of events (although some previous holidays were very short) called Micro-holidays.
Many events in World of Warcraft celebrate various holidays mostly based on those celebrated in Azeroth, but trappings of these holidays can also appear in Outland.
Patch changes
Edit to the right of the section title.
