Half Price Day is the third day of every month. The official neopets shops price their stock at half-price, and you can haggle down further from there. Speed is of the esscence on this day-- even more restockers than usual will flock to the shops to get a better price for the items and will be snapping things up faster than the younger players who tend to hobble into the shops at any ol' time. Kauvara's Magic Shop and the Post Office tend to be the most competitive.