Why is 在 translated as "during" in this sentence rather then "at"?
Don't use 'at the Dragon Boat Festival', that suggests to me a festival in a specific geographical location or place.
You could say 'at 5 o'clock' or 'at midnight', because those are specific times. 'At Christmas time' designates a specific time in the year.
在端午节这天，人们都喜欢吃粽子。
During the Dragon Boat Festival, everybody likes munching Zongzi.
or
On the day of the Dragon Boat Festival, everybody likes munching Zongzi.