Booster Shot Day is a special day held by Peach Creek Jr. High where everyone receives their booster shot. It is shown in the episode "This Won't Hurt an Ed" when Eddy and Ed announce that it is on that day as part of a gigantic prank against Kevin. While it is certainly possible that Eddy could have made up the day just to get back at Kevin, it seems that Booster Shot Day is an annual event, because Edd checks his notes to make sure that Booster Shot Day is actually weeks away. Because Edd says that the day is weeks into the future, it seems likely that Booster Shot Day happens in the late fall/early winter.
Although Eddy came up with the scheme in order to scare Kevin, Kevin is not the only one afraid of needles. In fact, everyone but Jonny and Edd were afraid of the day, so much so that Jimmy came to the event in armor and Nazz called her mom with a plain fear of death in her mind. These fears weren't helped by the rumors about the size of the needles going around school (a rumor assisted by Eddy and Ed) or the fact that Sarah had heard the needles could punch holes through truck tires.
It is unknown how often the day comes along, although it can be guessed that Booster Shot Day happens once every school year. The school nurse is known to be the one who administers the shots. The only consolation for the students is that after the shots are given, the patient is given a lollipop.