It's the date between the first and second date. This only applies when the second date has been meticulously planned and you don't want to bump it to the third date. This is the date where the magic happens.
Tina invited me out to the festival before our second date, I guess that's the 1 1/2th date as we had great plans for our second date.
When Bro's are phenomenal.
Dude, my best friend is bronominal.