Though the exact weather changes year to year, October is generally when fall temperatures start to become more apparent in New Orleans. Highs start off in the mid 80s and cool gradually to the mid 70s by the end of the month, with lows ranging from upper 60s to upper 50s. While the daytime will still be warm and pleasant, nighttime will start to get cooler, so if you plan on being outside, longer sleeves and jackets are recommended.
Skies are clear or mostly clear about half of the time, though there can be considerable cloud cover, which can take temperatures down closer to typical fall-like weather. Chances of rain hover around 30% to 35%, with near equal likelihood of that rain being light, moderate, or thunderstorms. Though the temperatures are decreasing, the humidity is not, and it remains in the range of mildly humid to very humid throughout the month.
Summary: October is arguably one of the best months to come to New Orleans in terms of pleasant weather and fun things to do, but keep an eye on the forecast, as temperatures can range from end-of-summer heat to a crisp fall cool.