what allie5 writes may be true for WDW....i'm not sure....
last time we went in the winter, we arrive on the sunday of thanksgiving weekend and all the decorations were already up as i recall....but i don't know when they began putting them up...
in the rest of the US, it used to be that decorations in public areas and in stores were up by thanksgiving.....but in recent years, as soon as halloween is over, the halloween decorations come down and the xmas decorations go up.....so it's possible that by nov. 18, some decorations would already be up (certainly in the stores outside of WDW....nov 18 is well into xmas shopping season in the US)....