Thousands of excited children and their parents are set to descend on Guildhall Square to see Santa switch on the Christmas lights — with a little help from Shrek and the stars from the panto Cinderella.
The fun starts from 6.30pm to 8pm, with the light up scheduled for 7.30pm on Wednesday.
There will be a festive and celebratory atmosphere as the city prepares for a bumper Christmas season.
Musical entertainment will come from the Chevys while BBC's Mark Patterson will be the host for the night.
Mayor Maurice Devenney said it was a huge family occasion for everyone.
He added: “The Big Switch On is a fantastic event.”
An allocated disabled viewing area will be available in front of the main stage.
Disabled access will be available via Guildhall Street with a drop-off at Queen's Quay Roundabout.