It sounds very glib, but the best time to book is when you find a price you can live with. For most fares that are not the bare bones, basic economy, if the price goes lower, you can sometimes get credit, and if the prices go up, well, you were smart, and locked in. We have 3 trips this year (none to Disney sadly). and our flights have been booked since December for our flights to Seattle in June. Our flights to Aruba in September have been booked since November, and our flights to London in October have been booked since January.
I keep an eye on them for price drops, and have already gotten some points back for the Seattle and London flights.
But, as others have said, if you have non-stop flights for 5 people for $1900 with times and airports that are convenient for you, that's pretty good.