What?!?!!? I grew up in Southern California and this will be my 3rd year in Denver, so I can't comment on other parts of the country, but it's ALWAYS been on October 31st. There are several organizations that I know of that host trick or treating in their locations, like different malls, and our medical center does an open house for trick or treating the weekend before Halloween, but it's not a "regional" or law enforcement thing - just something nice they do usually for advertising and promoting.
My DH did mention a week or so ago that one of his friends had "Facebooked" asking when Halloween was, Saturday the 30th or Sunday the 31st - apparently some people think it should be an "official" holiday celebrated on a specific day so more fun can be had without work or school the next day, or they feel that celebrating such an event on Sunday (a church day) isn't appropriate.
I just think that is the craziest thing on the planet!!! It's not a "real" holiday; we don't change St. Patrick's Day when the 17th falls on a Sunday to accomodate heavy drinking!! And if you feel it's inappropriate on Sunday, it should be inappropriate on ANY day (unless the only reason is because it's the Sabbath - in which case children learn about sacrafice and 'rest' and skip a year, not change the date!).
I'm curious to see if anyone else posts where there is an actual law enforcement order that specifies when it is.