I like that some festivals will be ruined by bad weather, but it should of course be announced.
Setting up a Sim Nation wide weather forecast should not be a problem, at least not within the current season. Weather itself might load upon arrival, but the matrix is already prepared. If you go to another world and check the calendar with weather forecast, then return 3 or 4 days later, it will not have changed.
There is something called surprise weather though, so I guess we would still need to deal with the risk of thunderstorms ruining festivals, but it would be reduced to those surprise days only. Besides, surprise weather often last for just a couple hours, not the full day. Guess we can live with that.