If I could predict when harvest would be, I’d be a rich man 😉
In all seriousness though, harvest typically happens sometime between September and October every year. The weather in spring and summer will have a huge impact on how the grapes ripen. Harvest time can also be one of the best times of year to visit — if the rains hold off, you’ll have some amazing vineyard views, and sweeping landscapes … and you might catch some wineries in the middle of production, getting an opportunity to see things that don’t happen year round. Also, keep in mind that Labor Day is a fairly high traffic wine tasting holiday, so that weekend can be busy.