to this day in 1990 when the Princess of Wales attended the passing out parade at the Royal Military Academy Sandhurst in a wide-brimmed Philip Sommerville hat. The addition of hatband stripes on the brim is a great touch.
Images from Getty as indicated
Royal Hats 
JamesB: She also wore it on Remembrance Day in Japan in 1990 – maybe its debut? These photos show how absolutely beautiful black can be! The sunshine really brings out the lovely details.
Embed from Getty Images
Sorry, I forgot to add this short video:
July 15, 1993: Ruth, Lady Fermoy funeral (Diana’s grandmother)
This is a strong look, which would still look good today (and you can’t say that for all of her outfits!)
If I recall correctly she also wore this for her fathers funeral in 1992.