《天工开物·上篇·乃粒》
浸稻种的日期，最早的是在春分以前，叫做社种，最晚的则在清明以后。
湿种之期，最早者春分以前，名为社种，最迟者后于清明。
The earliest date to soak rice seeds is before the Spring Equinox, which is called "Shezhong". The latest date is after the Pure Brightness Festival.