X Factor judge Simon is expected to join his fellow judges today at auditions in Manchester. They were originally scheduled to take place on Monday, but was cancelled following Julie's death.
Taking to Twitter last night, Simon said: "My family lost someone very special on Sunday. My Mum. I am overwhelmed with gratitude for all of the kind messages received. Thank you. X."
Julie passed away on Sunday (5 July) aged 89. A statement from the family said they were "heartbroken" at her passing.
Today, Simon will reunite with Cheryl Fernandez-Versini and new judges Rita Ora and Nick Grimshaw for the auditions.
It will be the first time fans will see the new judging panel together, and it will also mark the first official appearance of new presenters Olly Murs and Caroline Flack, who will be co-hosts.