What is the date/event that is the 'cutoff' for high school draft eligibility?

[rleb]

Example:
One draft year, players born 6/1/2005 to 5/31/2006 are eligible for the draft
Next draft year, players born 6/1/2006 to 5/31/2007 are eligible

Where does that June 1st cutoff come from?

For reference, here is how my league is set up :
In my universe, HS baseball is in the fall (Sept/Oct), and the draft is set for December 28th (so, you basically get drafted during your Senior year).

However, the cutoff seems to be somewhere around June 1st? Is there a league event that toys with that eligibility criteria? Or is it a set date?