Are you asking how busy the actual Night of Joy event is??? Or are you asking how busy it is in the park the day of Night of Joy??? Or are you asking how busy the property is because of night of joy???
If you are asking about the event itself, it is a hard ticket event meaning that you have to have a seperate ticket specifically for it so they are able to limit the number of tickets they sell. It is kind of busy during the event, but the majority of the people are there for the performers so the lines for the rides are not too long.
If your question is about the day guests in the park prior to Night of Joy, it is usually slow that day as the park closes early for the event, most people don't spend the day in MK, they go to a different park that has longer hours. So during the day MK is fairly slow.
If your question is how busy the property is because of night of joy, A LOT of the people that come for Night of Joy are locals or faily local, it is a big draw, but usually only for the night of the event, it doesn't really have an effect on the attendance for the week persay.