Moab's tourist season ebbs and flows. Back in the day, tourist would flood the town in the spring with the rest of the year being pretty slow and winter being almost completely dead. As the town has grown the tourist season has slowly expanded. The addition of many yearly events to the towns schedule (Moab Jeep Safari, Various downhill races, other Marathons, and The Moab Music Festival) also helped to expand the tourist season.
Currently the season starts to pick up towards the end of February and continues through November. During July and August we often see a reduction of tourist. Temperatures often reach near 110 degrees Fahrenheit which slows some of the tourist activity. Winter is still comparatively slow, but is a great time to check out Moab. During the winter some of the restaurants and shops shut down. The winter time often comes with some of the best rates to stay in Moab.
Early spring and late fall bring with some of the best temperatures to do vigorous outdoor activity like mountain biking or strenuous hiking. Depending on the year, these temperatures often carry into the winter.
As far as managing nightly vacation rentals, we often tell people to expect very little winter activity in the rentals. It's a great time to plan to use the vacation rental for yourself or to set up friends with an awesome place to stay.