The pair - who have a two-year-old daughter Harlow and 14-month-old son Sparrow together - are said to be planning to marry during the first week of December.
A source told X17online: "I heard Nicole Richie talking excitedly to Christina Aguilera about her upcoming wedding. She said, '29 days to go!'"
However, the nuptials will not be the only event the couple - who have been dating for over four years - will host, as they are also believed to be holding a "decoy event" to throw paparazzi off the scent of the real wedding.
Nicole's close friend Christina Aguilera - who an insider has revealed is "practically living at Nicole's house" - is also in talks with the pair to perform during their big day.
Meanwhile, it would appear the couple are keen for the newly single singer - who filed for divorce from husband Jordan Bratman three weeks ago - to get out and have fun.
Over the weekend Christina was spotted out with Joel, Nicole and Matthew D. Rutler, a set assistant from her new movie 'Burlesque'.
An onlooker told E! Online: "The group was hanging out in a corner of the private club together. Christina and Matt were cuddling on a couch together and left holding hands."