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In order to explain the observed features of blackbody radiation, Planck made a crucial assumption that the energy levels of oscillators within the blackbody are quantized. This means that the oscillators can only have discrete energy values rather than a continuous range of values, as was assumed in classical physics.

Before Planck's assumption, the classical theory of blackbody radiation predicted that the intensity of radiation emitted by a blackbody would increase without any limit as the frequency of the radiation increased. This discrepancy between the classical theory and experimental observations was known as the "Ultraviolet Catastrophe."

Planck's assumption of quantized energy levels resolved the ultraviolet catastrophe. He proposed that the energy levels of the oscillators are quantized in discrete steps, with each step corresponding to an energy level of E = nhf, where n is an integer, h is Planck's constant, and f is the frequency of the oscillator.

With the assumption of quantized energy levels, Planck derived a new radiation law that described the intensity of radiation emitted by a blackbody as a function of frequency and temperature. This law, known as Planck's Radiation Law, closely matched experimental observations and resolved the issues in the classical understanding of blackbody radiation. In summary, the assumption that allowed Planck to account for the observed features of blackbody radiation was that the energy levels of oscillators within the blackbody are quantized. This assumption resolved the ultraviolet catastrophe and led to the development of Planck's Radiation Law, which accurately described the observed features of blackbody radiation.