Windsor (United Kingdom) (AFP) - British pop legend Elton John will marry long-time partner David Furnish on Sunday, the ninth anniversary of their civil partnership, with a ceremony in Windsor, home of the castle beloved by Queen Elizabeth II.The 67-year-old singer posted a picture of an invitation on photo-sharing website Instagram, with the words "Sir Elton John and David Furnish request the pleasure of your company to celebrate their wedding on Sunday the 21st December" written on a background of red roses.Around 50 guests, including David and Victoria Beckham, Elizabeth Hurley and Ozzy and Sharon Osbourne, have been invited to the ceremony, due to take place at their home in Windsor, west of London, according to British press reports.Although known for his flamboyance, the ceremony is expected to be "a small, intimate do" with guests being served festive canapes and mulled wine, according to the Daily Mail.The picture of the wedding invitation was among the first three posts to his newly opened Instagram account.The others show a view of the couple's estate under the words "Good morning!