Princess Mary (25 April 1897 - 28 March 1965), was the daughter of George V and Queen Mary. This postcard was bought and written just before her marriage on 28 February 1922 to Viscount Lascelles.
(for lots more information, here's the wiki link)
Dear Little Gracie,
Although you have never seen this lady, she really does live, and in London. She is to be married Tuesday, and all school children here will have a holiday. Don’t you wish you were here to have the same?
Love & best wishes,