The global concentration of carbon dioxide in the atmosphere was measured at 400 parts per million for the first time in recorded history in May of 2013. It was a brief event at the Mauna Loa Observatory in Hawaii at the time. Within the next couple of years, however, readings of at least 400 ppm became standard.
The end of 2013 marked the first occasional observations of carbon dioxide levels in the atmosphere of 400 parts per million. There is nothing magical about that value, but we do tend to focus on round numbers.