Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
With 50,000 warriors on foot, and
20,000 on horseback or in chariots, the leaders of the Celts advanced
to the Apennines (529). The Romans had not anticipated an attack on
this side, and had not expected that the Celts, disregarding the Roman
fortresses on the east coast and the protection of their own kinsmen,
would venture to advance directly against the capital. Not very long
before a similar Celtic swarm had in an exactly similar way overrun
Greece. The danger was serious, and appeared still more serious than
it really was.
The belief that Rome's destruction was this time
inevitable, and that the Roman soil was fated to become the property
of the Gauls, was so generally diffused among the multitude in Rome
itself that the government reckoned it not beneath its dignity to
allay the absurd superstitious belief of the mob by an act still more
absurd, and to bury alive a Gaulish man and a Gaulish woman in the
Roman Forum with a view to fulfil the oracle of destiny. At the same
time they made more serious preparations.