We call it 'tight-assed Tuesday', where items (such as gas, movies, non-essential things - shopping items to a degree) basically lowers in price; by 10 cents or so for gas, while movie tickets lower from around $15 to about $8. I don't know the exact reasoning, but I figure it's so vendors can appear to be helping out people on Tuesdays by lowering their prices - despite them being so high to begin with.
I think the point of it is that its the least busy day, its the middle of the week and everyones working, so everythings cheaper to make you want to spend money. At uni tight arsed tuesday was always pizza night.