[...] "... Please correct me if I am wrong but, I think that Princess Margaret first wore a tiara at her sister’s Coronation on June 2, 1953 and then on June 8th at the Royal Opera House, Covent Garden during coronation week..."
However Princess Margaret Rose wore a tiara when she attended the (gala evening on the eve of the) Inauguration of Juliana as Queen of the Netherlands in September 1948.