Much like a wedding planner, most gala event planners start at least a year before the date. There are a lot of areas of focus in planning a large event and each has a sizeable action item list. It's incredibly helpful to have a document that outlines an overview of the main things that need to get done and when they need to be done by.
Start on your first day of planning, which ideally would be at least 12 months out, and make sure all of the main points are addressed in monthly increments right up to the day of the event. As you get closer and more details need to be addressed, you may want to break it down into a week-by-week timeline.