Tadcaster in North Yorkshire was split in two on Boxing Day, when the River Wharfe bridge partially collapsed.
The bridge is now due to be finished by 27 January rather than Christmas, after "unusual and persistent" high river levels caused delays.
The arch rings mean stonemasons can start work covering the arches.
David Bowe, of North Yorkshire County Council's Business and Environmental Services, said the arch rings had been lifted in a day early.
He said: "Last night, we managed to lift in the large formwork arch rings which will allow us to get on with the masonry for the arches on Tadcaster Bridge.
"Next the masons will start constructing masonry piers and filling them with concrete, and putting masonry over the arch rings.
"Our new target completion date is 27 January, a month after the original date, but the guys on site are really up for the challenge and determined to beat that date."
New piling and construction of the pier base, the most complex part of the operation, are complete.
Piling would help prevent future scouring of the foundations, which contributed to the bridge's collapse last year, the council said.
The government pledged £3m for repair work within days of the bridge collapse, and £1.4m was given by the region's Local Enterprise Partnership to widen and strengthen the bridge.
Arch supports have been put in place on a Grade II listed bridge damaged by floods last December, enabling the final phase of masonry work to begin.