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’ Was a Big Deal, After All