Date Availability
Rodeo dates are listed on the schedule as soon they become available from the official sanctioning body (such as the PRCA or PBR). Unlike other professional sports, dates for particular rodeos are determined by individual rodeo committees. Committees must submit their dates to the sanctioning body and have them confirmed, and available dates may depend upon local conditions.
For example, a PRCA November rodeo may not confirm or lock in their dates with the PRCA until July or August of the same year. On the other hand, the PBR has most of its dates pretty well locked in (at least on their top-level tour) but frequently update their other tours in much the same way as the PRCA does, as the year goes on.
Confirmed Dates
Rather than leaving up the previous year's dates (and causing confusion), I've decided to remove the dates until the actual confirmed dates are available. This is why some dates are there and others are not. I try to update the rodeo dates every couple of weeks but there maybe some lag time between when the committees lock in their dates and when they are available to the media.
Larger rodeos, such as Rodeo Houston or the Reno Rodeo, tend to have confirmed dates in advance due to their high profiles and size. Smaller rodeo tend to try to keep the same dates every year but may move their dates in rare instances so as not to conflict with other local festivals, events, etc.
Planning Your Trip
For those of you traveling and planning trips to particular events in advance, I know this can be difficult. But if a rodeo has been held the 2nd week of June (for example) for years, it's a safe bet that it will remain at the same time of year. I'd say 80% of all rodeos keep the same dates and only in extreme circumstances are significant changes made.
So if dates are missing from a particular rodeo, check back as the rodeo gets closer and they will be posted as soon as possible.