The New York-set Bollywood love triangle, which also stars Saif Ali Khan, originally released on November 28, 2003.
Karan Johar said: "9 years to Kal Ho Naa Ho... the last film my father helmed at Dharma productions... an ironic title of a very special [film] to me... always..."
Fans of the iconic Bollywood film shared their favourite scenes from the movie with Digital Spy on Twitter.
"Favourites would have to be when Naina (Zinta) finds out truth of Aman (Shah Rukh Khan). So sad. Also, when she goes to confide to Aman her love for him. Lastly, the ending dialogue of Naina Catherine Kapur sticks with you and makes you sad just thinking of it," said Akaash Kumar.
"I like the collage after 'Kuch Toh Hua Hai' and Sweetu's (Delnaaz Paul) love story. Oh and the galat ghar sequence," posted Ekta Valecha.
"My fav scenes: 1) Aman teaches Naina to smile. 2) Naina asking Aman why he loves her so much," shared Saleha.
Shamaila added: "Mine had to be the scene when Preity gets married, the song, both her and Shah Rukh crying! *sobs*."
Bollywood academic and Pink Rupees or Gay Icons? writer Charlie Henniker said: "For me it's definitely the moment Shah Rukh and Preity first encounter each other without knowing it - it still sends a shiver down my spine.
"The KHNH theme music has so much resonance from that moment."
The Nikhil Advani-directed film won two National Film Awards and seven Filmfare Awards.