What do you mean it's colder than the day Zhen Xuan went to Ling Yunfeng from Kanroji?

The sentence "It's colder than the day when Zhen Xuan went to Ling Yunfeng from Kanroji" means that the weather is very cold, even colder than the day when Zhen Xuan went to Ling Yunfeng from Kanroji.

In the TV series Empresses in the Palace, Zhen Xuan went to Ling Yunfeng from Kanroji, and the wind was cold. Zhen Xuan, who had been infected with the cold, almost died. Fortunately, King Guo County appeared in time, which made Zhen Xuan have a life in Ling Yunfeng.

Therefore, the phrase "colder than the day when Zhen Xuan went to Ling Yunfeng from Kanroji" is used to describe the unusually cold weather.