The actress revealed to Digital Spy that the episode will air in July ahead of the London 2012 Games.
She also confirmed that the Ab Fab special will see her character Patsy gain access to the Olympic Stadium in Stratford with Eddy (Jennifer Saunders).
"It is going to be shown I think the week before the Olympics start," she said. "Bubble (Jane Horrocks) takes it to heart, Eddie and Pasty... we do get into the Olympic Arena and for one magic moment we feel that we might have been two Olympic heroines."
Speaking at the launch of Marks & Spencer's Shwopping campaign, a clothes recycling initiative aimed at reducing the 1 billion items dumped in the UK each year, Lumley added: "Eddie and Patsy probably wouldn't Shwop, but Jennifer and I would Shwop."
Absolutely Fabulous returned for two specials on Christmas Day 2011 and New Year's Day. Saunders has confirmed that a movie is also in the works.