Look, it's a thing now, okay? You're on the internet, you have to expect people just making shit up and slapping 'day' on the end so it goes social. October 21st 2015? Back to the Future day, sure.
Whether you celebrate dubious made up calendar events or not however, you have to appreciate just how well done this tribute from GTA Series Videos is. And obviously, there's only one Grand Theft Auto character that could do Doc Brown justice:
If you're playing on PC then you can grab the mod here.