The arrival of her monthly period indicates that the young wife has finally grown up. This is a signal, and logically it should be a greatly joyous occasion.
"Have you held your coming-of-age ceremony?"
In the Four Continents, girls have their coming-of-age ceremony at sixteen. He knew the little girl was already past fifteen but forgot to ask about this ceremony.
Jiang Xue knows that ancient people valued the coming-of-age ceremony, as it marks a girl's transition into adulthood, but she is a modern person and hasn't considered it.
Moreover, she hadn't celebrated her birthday since crossing over, nor did she know when she was born. Dong Yao mentioned it in passing earlier, but she had forgotten, and she wasn't planning to do anything about it now.
