Zongzi are eaten on June 14 during the Dragon Boat Festival. It is said this is to commemorate Qu Yuan, a poet who commited suicide by drowning hiimself in a river. The locals got on boats to save him, but when they realised it was too late, they threw rice dumplings wrapped in bamboo leaves into the river to stop the fish from eating his body.
References: https://www.scmp.com/magazines/style/well-being/article/3136943/why-do-we-eat-zongzi-during-dragon-boat-festival-history