April 21, 2015 was dubbed "Mobilegeddon" because it was the day Google launched the algorithm shift which include mobile usability as a ranking factor. Some analysis suggested there was little actual impact and Mobilegeddon was a misnomer. However a more recent study indicates that the algorithm shift did have a notable impact. It just took longer to be detected than initially thought. For more, see this WSJ article featuring a study by Adobe Systems: Google’s ‘Mobilegeddon’