Bang Day is the day that you first have sex with the partner of your new relationship. Bang Day does not refer to one-night stands - it is relationship dependent. If your relationship lasts longer than one year, Bang Day then becomes known as your Bangiversary - the anniversary of the day you first had sex with your partner.
"Hey man, how long you been seeing Sarah?"
"Two months - yesterday was our Bang Day."
"Nice, congratulations dude."