That most amicable of divorced couples, Fergie and her ex-husband Prince Andrew, plans to celebrate his 40th birthday together in a weekend of parties starting Saturday, Feb. 19. The merriment is set to culminate in a 200-guest society bash under the largest roof in the world, London's new 20-acre Millennium Dome. Among the festivities: an intimate dinner with close friends on Feb. 19, followed the next day with Fergie and Andrew's attending a pro-celebrity tennis tournament in London. (Players will include former Wimbledon winner Jana Novotna, British tennis player Tim Henman, Goran Ivanisevic and singer Sir Cliff Richard.) The sports event at the London Arena will raise money for the Duchess of York's foundation, Children in Crisis.