Foregoing any GTKY'ness
'Graduation Day' is generally applied to schooling, meaning the day that your course requirements are completed and you are issued your diploma or otherwise and then sent on your merry way. There is usually some pomp and circumstance to go along with it to commemorate the time spent in said institution.
In north America and most of Europe it is usually applied to 'High School', though it also can apply to College/University or Trade Schools.
The most well-known thing about Graduation, beside being dimploma'd, is the ever-present mortar board.
Now that that's out of the way... My High School Graduating class, in '97, had 800 graduates. About 4/5 of the class gave To our Principal, as we accepted our diplomas, one penny. Economies of scale, baby.