Before anyone asks...
2011: 20 Halloween parties: Sept - 6; Oct - 13; Nov - 1
2012: 22 Halloween parties: Sept - 6; Oct - 15; Nov - 1
2013: 23 Halloween parties: Sept - 7; Oct - 15; Nov - 1
2014: 26 Halloween parties: Sept - 10; Oct - 16
2015: 25 Halloween parties: Sept - 6; Oct - 18; Nov - 1
2016: 29 Halloween parties: Sept - 11; Oct - 18
2017: 32 Halloween parties: Aug - 2; Sept - 11; Oct - 18; Nov - 1
2018: 34 Halloween parties: Aug - 4; Sept - 12; Oct - 18
2019: 36 Halloween parties: Aug - 5; Sept - 12; Oct - 18; Nov - 1
I'll edit later to break those down by parties/month.
EDIT: Updated the info above. October has been steady at 18 parties since at least 2015, and at least 15 parties since 2012.