- 19.036 on June the Twenty-second
The first day of summer, give or take on a leap year.
Twenty, combined with all the ordinals or cardinals below ten, takes the hyphen according to OED.
“twenty, adj. and n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/208007. Accessed 21 September 2017.