There's actually two dates. One for the distributors and a later date for the retailers to be fully switched over.
from 2013 https://www.eia.gov/todayinenergy/detail.php?id=11031
but looks like its still valid.
https://www.caranddriver.com/news/the-vapor-rub-summer-versus-winter-gasoline-explained

The winter fuel RVP is a culprit in the warm early spring weather vapor lock.
What to do about Winter fuel in the summer?