It traditionally rains on the Chi Hsi Festival, the Chinese Valentine's Day, on the seventh day of the seventh lunar month, because that's when the Weaver Girl weeps for having to part from the Cowherd Boy. And that's why you're supposed to make a little hole in the tang-yuan dumplings youeat that day.
七夕總會下雨，因為織女為要別離而掉眼淚，所以七夕吃的湯圓要挖個小凹洞。