Why and how is the bartholomew day massacre significant in the course of the french wars of religion?

Answer 1

The Catholic verses Protestant squabbling turned massively violent. Protestants were murdered in large numbers. (No one knows exactly how many.) Many Protestants left France, including one of my ancestors on my mother's side who moved to a neighboring German principality near Heidelberg and continued his brewing business. Long long of beer brewers there.
Some answerers will say the migration of Protestants out of France occurred113 years later in 1685 when Louis XIV revoked the Edict of Nantes - an agreement tolerating Protestants in Catholic France made in 1598. Up to 200,000 French Protestants did leave after 1685, but some - like my old relative - left in 1572 after the Massacre.
The pope in 1572 was Gregory XIII - the guy who changed the Julian calendar to the one most of us use now - the Gregorian calendar. Pope Gregory celebrated the massacre of tens of thousands as a good thing for the Roman Church.