It's safe to assume that Thanksgiving is the one day of the year where there is the greatest number of races, most of them 5Ks. This is especially true in a city as large as Houston, the third largest city in the nation. The good thing about that is that nobody has to wander far from home to work up an appetite for the Thanksgiving dinner later on.
Generally speaking, these are not events to PR at. There are a huge number of walkers, and for many of them, this is the only event they do all year long. It really tends to slow down the field, but hey, it's all about having fun, right? For a full listing (at least the ones I've been able to find) of local turky trots, check out the interactive race calendar.
This year Thanksgiving falls on a Thursday, which is of course November 25. After picking your race, the only thing left decide is what the main course will be. Turkey? Ham? Spam?