Stay on rainy days, but stay on days. It means that the host doesn't stay, and the guests have to leave.
If it rains, stay for a day, or not? Stay. The host keeps the guests, and the guests don't have to go.
This sentence comes from a story: Xu Wei (Xu Wenchang), a famous painter in the late Ming Dynasty. It is said that Xu went to his relatives and friends' home when he was poor, and lived on his own without food. Once, it happened that the spring rain continued, and the family he sent food to was very boring. One day, when the host saw Xu Wei going to the toilet, he wrote a line on the wall and politely ordered him to leave. That line goes like this:
"Stay in rainy days, stay in days"
When Xu Wei came back, he naturally understood the meaning of the words, but instead of leaving, he smiled and said, "Since you are so kind to keep me, I will stay again!" Say, raise a pen and circle this line:
"Rainy days, stay a day. Will you leave me? Stay. China cuts copper and expands it. "