LONDON — Kate, the Princess of Wales, attended the men’s final at Wimbledon on Sunday, in a rare public appearance since she announced her cancer diagnosis earlier this year.

A big tennis fan, Kate, 42, has been a fixture in the Royal Box for many years, often attending with her husband, Prince William, and other members of the royal family. This year, she was joined by her daughter Charlotte, 9, and sister, Pippa Middleton Matthews, 40.

Wearing a long purple dress, Kate smiled and waved…