Due to recently-discovered technical issues with a separate satellite, ESA has decided to perform additional precautionary verifications on its Gaia satellite. Therefore we have requested that Arianespace postpone the Gaia launch, currently scheduled for November 20, 2013.
More details will be given as soon as they are available and the new launch date will be announced when the timeline for completing the additional work has been confirmed.
Updated 29 October:
The upcoming launch manifest of Arianespace has now been established. Gaia is scheduled for launch on 20 December.
For additional information on the postponement, see latest update on the ESA Portal.