"Sylvan" pertains to woods, the forest, or a pastoral landscape. Keats' urn depicts various people and circumstances: a religious ceremony, two lovers, a pastoral piper, townspeople, and the forest itself.
These images "occur" in a woodland (sylvan), rustic scene.
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare; (15-16)
These images "occur" in the sense that they happened in the past and are frozen in time on the face of the urn. This is what the speaker is contemplating: the transitory nature of life - immortally frozen in time as a work of art. Therefore, the urn is a historian; it has recorded past images of life in a woodland setting. The urn is a "Sylvan historian," an object that has chronicled and archived an image of rustic life from the ancient past.
Important to the poem's themes of life, time and immortality, the urn has captured a moment when the sylvan scene is full of life: life of the people and of the forest itself:
Ah, happy, happy boughs! that cannot shed
Your leaves, nor ever bid the spring adieu; (21-22)
The leaves, in spring, are blossoming. Life is emerging. As the scene is frozen in time, it will always be in this state of emergence.