I can't give a specific date as to when it happened, but I assume it would be any time after the Isstvan V Dropsite Massacre. After that "battle", several World Eaters started painting their armour red and bronze. As the Heresy progressed, more and more did the same until the Legion's colours were no longer the white and blue of the War Hounds. I believe a few also began to mark their armour with Khornate icons as well. Therefore, it would make sense that they started adopting the "bunny ears" around this time too.
The main point though is that it was a gradual thing; there was no set day when the whole legion got together, had some tea and cake, then set about carving the bunny ears on, and finally slaughtering millions of people to paint their armour in their blood. World Eaters just don't eat cake. And they certainly don't have grand reunions in the name of Cake.
“Captain Saul Tarvitz had faced storms of gunfire and monstrous aliens and even glimpsed the insane predators of the warp, but when Angron charged, he ran.”