Well out of the 7 times a 31st appears, 2 of them are holidays, Halloween and New Year's Eve. I would have guessed that this might increase ticket issuance, but I suppose it's possible it could have a negative impact as well.
[1] https://data.baltimorecity.gov/Financial/Parking-Citations-2...
Maybe, there's a possibility of increased ticket rates too due to the extra driving km done by people "going away for the long weekend", and by drivers being less experienced on the routes they're driving (compared to regular commuter traffic who know exactly where the speed/redlight cameras are, and are usually in heavy enough traffic to not be able to exceed the speed limit).
I wonder if there's local (NSW, Australia - for me) tickets-by-day day available to analyse?
maybe a better question would be, for ever hour worked, how many ticks are issued. aka normalizing against the number of patrol men/women active for a day or an hour.
e.g. more police on patrol == more tickets, but does not mean each police man/woman is biased to issue more tickets near the end of the month to meet their quota.
also consider counting backwards. e.g end of the month = 0, 1 day before end of month = 1, 2 days before end of month = 2 etc..
my 2cents.