The checks on the Gaia satellite are proceeding nominally, enabling Arianespace in agreement with ESA to announce the launch date. 

Liftoff of the Soyuz ST-B launcher is set for Thursday, December 19, at precisely:

09:12:18 am (UTC)

06:12:18 am (local time in French Guiana)

10:12:18 am (Paris)

04:12:18 am (Washington, D.C.)

12:12:18 pm (Moscow)