While different critics take varying views of the overall unity of Keats' text, the textually logical explanation of the different scenes that he describes is that all are different parts of the wedding ceremony that has been captured on the urn. It would help if we knew the functional purpose of the urn. Grecian urns were at times used as funereal urns for the ashes of the cremated dead, though they were also used for many other purposes, like to hold water or wine.
Some speculate that perhaps Keats saw the urn in the British Museum when he was there in 1816 and knew the purpose of the urn. If we suppose that the urn was used as a funereal urn for the dead, then we are led to the speculation that it must have been the bride or the groom who died before the fulfillment of the wedding and the celebratory actions attending the wedding, like the sacrifice to Hymen and a procession of guests from the town. It is equally possible that the urn was used as a wine urn for the wedding celebration, in which case the artist may have wanted to capture forever the tender moments before the fulfillment of the marriage.
The wedding Keats describes in the scenes of the "leaf-fring'd legend" was held out of doors. Though Keats knows nothing more than what he sees on the urn, he wonders if it might have been held in the northeastern seacoast Vale of "Tempe" or in the "dales of Arcady," located in southwestern Greece. Keats describes a wedding procession led by a priest--probably the priest of Hymen, the Greek god of weddings--who leads a sacrificial young cow and who is followed by the guests from the nearby town. The bride and groom are serenaded by "timbrels and pipes" and are still poised at a distance from each other in the scene with the wedding unconsummated.