Were that the case it would certainly make planning a lot easier, but sadly it's not. In 2008, the parties started on the first Friday in September (so very near the beginning of the month) and were held every Friday and Tuesday that month, with a random Sunday thrown in. In 2009, they again started on the first Friday and followed the Tuesday/Friday pattern, but with one skipped Tuesday. In 2010 they didn't start until the second Tuesday (mid month) and there were no Friday dates at all (3 Tuesdays, 2 Thursdays, 2 Saturdays). 2011 saw a return to the the Tuesday/Friday format, but again not starting until the second Tuesday.
2008
Sept:
Tuesdays 9th, 16th, 23rd, 30th
Fridays 5th, 12th, 19th, 26th
Sunday 28th
Oct:
Monday 13th
Tuesdays 7th, 21st, 28th
Thursdays 2nd, 9th, 16th, 23rd, 30th
Fridays 3rd, 10th, 24th, 31st
Sundays 5th, 19th, 26th
2009
Sept:
Tuesdays 15th, 22nd, 29th
Fridays 4th, 11th, 18th, 25th
Oct:
Monday 12th
Tuesdays 6th, 13th, 20th, 27th
Thursdays 1st, 8th, 15th, 22nd, 29th
Fridays 2nd, 9th, 23rd
Saturday 31st
Nov:
Sunday 1st
2010
Sept:
Tuesdays 14th, 21st, 28th
Thursdays 23rd, 30th
Saturdays 18th, 25th
Oct:
Monday 11th
Tuesdays 12th, 19th, 26th
Thursdays 7th, 14th, 21st, 28th
Fridays 1st, 8th, 22nd, 29th
Sundays 3rd, 17th, 24th, 31st
Nov:
Monday 1st
2011
Sept:
Tuesdays 13th, 20th, 27th
Fridays 16th, 23rd, 30th
Oct:
Mondays 10th, 31st
Tuesdays 11th, 18th, 25th
Thursdays 6th, 13th, 20th, 27th
Fridays 7th, 21st, 28th
Sundays 2nd, 16th, 23rd 30th
Nov:
Tuesday 1st
There are some patterns. There's always a party on Halloween itself, and always a party on the second Monday of October (Columbus Day). There are no Wednesday parties. In October, at least, I suspect they start with the two fixed dates and work around that. No doubt someone with some time on their hands could work out the formula. This year, though, Halloween falls on a Wednesday so I wonder whether that will throw things out?