I was literally pondering about the same thing
Actually the name of the manhua came from a project they were working on. Old Xian including other 18 artists decided to draw/tell a story. each artist did their own story, and altogether did 19 stories, and one of the stories is old xian story. old xian liked the story she worked on, so she decided from this one shot she did for this project to make a story out of these characters which led to what we have now. those 19 stories project name was called "19 days" so when she developed her characters she stuck to the name 19 days. So, to me at least, I feel the name was lost on this manhua, I think she would have called it one day maybe since we see that written on top of the page.
ohhh yeah didn't know that tqqq
anytime ^^"
its been like 3-4 years since I started reading this but only now have i begun to realise I've no clue what the title means. 19 days? idk if its mentioned in the story and I jus forgot but yeah