In that case, the telescope will lift sometime between 13.20 and 13.52, Swedish time.
The Web telescope was initially scheduled to be launched on December 18, but space travel has been delayed several times due to various circumstances.
James Webb, named after a former Nasa chief, will be a replacement for the Hubble Space Telescope but, unlike its predecessor, will not be in orbit around the earth - but the sun.
The project began in the 1990s and initially it was hoped to get the space telescope off already in 2007.