The former Girls Aloud member said that Sunday was Harding’s day as this was the day of her death and when her “spirit was free to fly unrestricted”.
Coyle shared a black-and-white photo to Instagram of Harding with her arm outstretched and smiling brightly, with the caption: “Reaching back at you beautiful girl!!!
“Sunday is Sarah’s day!! We lost you on a Sunday. Heaven gained you on a Sunday & your spirit was free to fly unrestricted on a Sunday. I feel you nearby on this Sunday!!”
Cheryl posted a video on Instagram featuring multiple clips of the girl group and Harding at live concerts, award ceremonies and chat shows.
She captioned the video: “Memories may be all we’re left with, but they’re enough to last a lifetime #Sarahs40th.”
Roberts said she wished that the pair could have “run riot like we used to” to mark the big occasion.
“I know you’re somewhere making trouble live one,” she wrote in the caption of a photo featuring her and Harding, who is showing her middle finger.
“Happy Earth birthday Sarah.
“Love, miss and thinking about you all the time.
“I feel your energy like you’re still here.
“Wish we could have ran riot around town like we used to for your big 4.0 today! Cause a supernova up there electric girl.”
Walsh also shared images of her and Harding laughing together and said he had spent the day reflecting on the memories she has of her former bandmate.
She wrote on Instagram: “Today is a very strange day.
“We have celebrated so many birthdays together over the years so this feels very surreal.
“I hoped with all my heart that we would be celebrating our 40th’s together too as we planned.
“You always brought the party and I have spent today reflecting and reliving all the amazing memories we made.”
Girls Aloud were formed on TV show Popstars: The Rivals in 2002 and went on to produce five studio albums during their time together, which included the hit tracks Sound of the Underground and The Promise.