This date was probably chosen as Drive in Movie Day because it was on 6 June 1933 that the first one opened, on Admiral Wilson Boulevard in Pennsauken Township, New Jersey. The screen was 40 by 50 ft (12 by 15 m) and there were spaces for 400 cars. The advertising slogan read "The whole family is welcome, regardless of how noisy the children are."
10 things you didn't know about drive in movies:
https://topicaltens.blogspot.com/2024/06/6-june-drive-in-movies.html