Each plant and utility will have a different outage schedule and length depending on a number of factors, to include fuel burnout (i.e. cycle length) and expected maintenance/projects.
Generally there is a fall and spring outage season for nuclear plants. With the statement above, you can expect the months to be fluid. But generally a utility will try to get their outage in when power production demands (and replacement power costs) are lower. Those would normally be in the fall and spring, since winter has higher heating costs and summer has higher cooling costs.
Normally, August-November/December and January/February-May would be your windows for these seasons.....