Spring and autumn are the obvious seasons: cool enough for the long walks through Villa Adriana and Villa Gregoriana, and the fountains of Villa d'Este are at their best when the water is high. April and May are particularly fine, with wisteria and lilac in flower in the gardens.
July and August are very hot in the open archaeological park of Villa Adriana — start as early as possible. Winter is quiet, sometimes cold and damp, but the gardens of the Villa d'Este are still attractive on a sunny morning.