The fictional last day of the month of June. Generally to be used as an ironic statement in reference to an event.
My birthday is on June 31st.

I'll pay you back after the 31st of June.
by Erick Landrider March 19, 2009
Get the June 31st mug.
A special extra day of Pridemonth, where Superstraight pride is celebrated.
Boy, it's the 31st of June - and superstraights are so valid!
I've stopped being superphobic ever since June 31st.
by q_Maxi_p June 6, 2021
Get the June 31st mug.