If you need to change the weather for an important festival or for another event, try the following. Save your game before going to bed in the game. When you wake up in the game, if the weather isn't as you wanted it, reload your game, go to sleep again, wake up in the morning and see if the weather is what you wanted it to be. Since the weather is randomly determined, you can repeat this as many times as you want until you get the weather conditions desired.