"Sarah asked us to do this and it’s really important for us that we make it a big event."
The band hope to raise money in Sarah's name, following her death from cancer last year at the age of 39.
“Although we knew this day would arrive I am somehow still feeling at a loss for words."
'My Heart Is Broken': Kimberley Walsh Pays Tribute To Former Bandmate Sarah Harding Following Her Death
The singer and actor said Sarah's "infectious chuckle" was “one of my favourite things in the world”.
Sarah's fearlessness, shining personality and, of course, talent helped propel Girls Aloud to stardom.
The singer's death was announced on Sunday by her mother.
Geri Horner and Davina McCall – who met Sarah back in her Popstars: The Rivals days – are among those to pay their respects.
The chart-topping star revealed last year that she'd been diagnosed with breast cancer.
The singer celebrated turning 38 with her bestie and former bandmate, Kimberley Walsh.
'Every Little Victory Feels Momentous': Sarah Harding Says Tumours Have Shrunk After Cancer Treatment
The Girls Aloud star says she was given "a little bit of positive news" at the end of last year.