The Jacaranda Festival is the one time Grafton becomes more than a pleasant river town. Streets and parks bloom purple in late October and early November, and the town builds a full civic celebration around the trees: parades, markets, music, and the kind of local pageantry that only works when a place genuinely owns it.
If you are choosing when to visit, this is the easy answer. Book ahead and allow time to walk rather than just photograph one avenue from the car.