What social event was added to Maycomb's social calendar, and why was it created?

I think that you are talking about what happens in Chapter
27.  This is when the ladies of the town decide to have an organized Halloween event at
the high school auditorium.

There was going to be a pageant
and there would be various games and there was going to be a contest for the best
costume, as long as it was made by the person wearing
it.

This was done because there were worries about crime. 
This was especially true after the Barber sisters had their home burgled.  Because of
this, the ladies seem to have thought it would be better not to have the kids out
running around.