Lesbian slang used to identify other possible lesbians. If a lesbian is into you but isn’t sure if you like girls, she’ll tell you to have a happy/good/etc. earth day.
Lesbian #1: hey girl hope you have a good earth day!!
Lesbian #2: omg thanks!! hope you have a good earth day too!!
"Hey, do you know who The Person That Made Earth Day is?"
"Yes. You are a funny person"