Why 19 days? Is everything happens in 19 days?

that's not the reason for sure, but i honestly don't know what the real reason is (the stroy happens over some years)

originally 19 days was a book which 19 artist (including Old Xian) created one-shots it was "one day" or chapters to the artist collaborating in it. I think Old Xian came up with the name from then.