It is resort by resort so you'll have to call to get the specifics. As a generalization, assume any time that's red time will be Mid or High and any time that's peak and red will be high. But as another poster proved this week, many people aren't really aware of what's peak and what's not. For Four Seasons I suspect summer will be high and at least most of the rest will be mid season. For Aruba, I suspect it's week 51-18 will be High for the Marriott and the rest mid. It's possible some of the truly off season for Aruba is low like Sept and Oct. Remember that it's actually the destination resort that determines the season, not
DVC.
Unless you don't have enough points or are truly flexible on your travel and just trying to pick the lower times, I doubt it truly will make much difference for most members most of the time. The difference from low to high is not that much. There's also the Travel Index on II's resort directory that may help you further. Otherwise, just email or call DVC and ask for specific resorts.