Times it is appropriate to decorate/advertise for holidays

 New Year's Eve/Day

The day of New Year's Eve, if desired. Stores may begin advertising their New Year's Eve decorations on December 20.

Valentine's Day

February 1. The holiday occurs in the middle of the month; that is enough time.

St. Patrick's Day

March 1. Ditto.

Easter

March 18. This one is tricky because Easter floats around on the calendar, so to speak. 

Cinco de Mayo

I actually do not want anyone who isn't Mexican to celebrate this day. It is a thinly-veiled excuse to get hammered and act racist. Celebrating by listening to Mexican music and/or dining at local Mexican-owned restaurants is fine for everyone. Do not celebrate, decorate, or advertise by committing cultural appropriation. Advertising may begin on April 20, but NO ALCOHOL ADVERTISING. Not even Mexican alcohol.

Mother's Day 

May 1, although I don't think anyone actually decorates for this (unless you are having a mother's day party, in which case, the day before or day of is fine). Advertising: April 20. 

Father's Day

May 20 (advertising). No one really decorates for Father's Day. 

Independence Day (Fourth of July) 

Decorating: the last week of June. Advertising: the day after Father's Day.

Halloween

Both decorating and advertising can begin no earlier than October 1. The holiday is at the end of the month; there is no need to decorate or advertise for Halloween in September. Generic fall decorations may be used from September 1 through November 30; explicitly Halloween/spooky-themed decorations must be taken down the first week of November (thanks to Día de los Muertos). A loophole exists thanks to The Nightmare Before Christmas movie; decorate your house with decorations from or inspired by the movie, and it can do double duty for Halloween and Christmas. I don't really like or approve of this, however.

Thanksgiving

Decorating: November 1 for explicitly Thanksgiving-themed decorations (generic fall decorations may be used from September 1 through November 30). Advertising: November 1.

Christmas

Decorating: the day AFTER Thanksgiving is fine, for your personal home. December 1 is preferred. November 1 is NOT acceptable; let fall and Thanksgiving have its due. I wish people would stop acting like people who hold fast to the no-Xmas-decorating-until-after-Thanksgiving rule are joyless fun-suckers; we are merely temperate, principled people who know the value of keeping holidays in their rightful place. Advertising: I don't want to see a single fucking Xmas tree/Santa/present/reindeer/snowperson or hear a single bleeding carol until December 1. I don't give a shit about Black Friday or your bottom line. Everybody knows when Christmas is and when and how to buy Christmas decorations and presents. Advertising interference is not needed. Every year a local radio station starts playing Christmas carols earlier and earlier; last year I emitted a howl of rage when I turned on the radio station on November 15 (NOVEMBER FIFTEEN!!!) to hear fucking Jingle Bells. The madness needs to stop. 

"The Day After Thanksgiving" by Brandon Heath

Christmas decorations can stay up until January 6, Ephiphany/Three Kings' Day.