adj. metonym. A day spent evading from one's otherwise usual responsibilities (work, school, babysitting etc) with one or more other people, in the pursuit of a more fun activity - typically drinking, vandalism of driveways and other such anti-social behaviour.
Inspired and popularised by Episode 2 Series 1 of the Channel 4 (UK) television show The Inbetweeners.
Me and the boys had an inbetweeners day last Thursday - it was great, we beat all the queues at Thorpe Park.
Dangerous Dave: Wagwan bruv, com'e'go do an inbetweeners day tomorrow.
Sensible Steve: No man, we got a conference call with the Washington office in the afternoon.
Dangerous Dave: Allow that shit man, we'll go Thornton Heath and chirpse some clunge innit.
Sensible Steve: Aight, safe.
Greg did not approve with the group's decision to have an inbetweener day on Thursday. What a bumder.