Plans can only be created for dates 365 days from today. That's because our Crowd Calendar only projects for the next year; after that the accuracy suffers a bit.
In the meantime, you can create plans for an equivalent date within the next 365 days--any sort of planning done this far out will be very rough, regardless, and the crowds and showtimes should be pretty similar.
Once your travel dates are within 365 days, you can adjust your plans to use the real dates and re-optimize. We also encourage you to re-visit your plans close to your travel dates in order to take advantage of the latest show times, wait times, and other information.