June 31st Share definition Flag 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. June 31st by Erick Landrider March 20, 2009 0 0 Get merch Get the June 31st mug. Buy Now — $32.95 See more merch