Does anyone know if there are nightly fireworks at MK, or only on special nights? Thanks!

To my knowledge they are every night. If want to know specifics you can go the WDW site and look at the calender for when you are going. Click on the date you are interested in and it will give you the days events and hours.

There are times of the year that they don't run them every night, mostly, I've noticed, when there's a special hard-ticket event that night. (Mickey's Halloween or Christmas parties in particular)

But the calendar on the Disney site is usually accurate.