We wont know what the weather for gates open will be until the Monday of the festival. Maybe Sunday before. And we won't know what the weather for Saturday/Sunday of the festival will be until we're actually on site.
But that doesn't stop people starting threads a year in advance.
As for the info, in a few weeks someone will open the gates of Broadmoor and Rampton and give the escapees internet access to a weather forecasting model for them to post on here in between meals where they eat their family members spleens.