When does 秋分后遇壬/逢壬 refer to? I'm working on the, topolectical, word 出沾.

《四川方言词语续考》explains it as:


  秋分後遇壬叫入霑，十日滿叫出霑。


李實's《蜀語》defines it as;


  秋分後逢壬謂之入霑，十日滿謂之出霑○霑謂雨多也。逢壬十日內謂之霑天。諺云： ‘入霑有雨 出霑晴。’


Which is pretty much the same.

The part I don't get is the whole 遇壬/逢壬 part.

壬 could be talking about 壬年, but in the same manner it could also refer to 壬日 and I suppose 壬月 as well.

When exactly is 秋分后遇壬 or 秋分后逢壬?
It refers to 壬日. 

So 秋分后遇壬 means after 秋分, from the 1st 壬日 霑 starts (i.e. 入霑), after 10 days 霑 ends (i.e. 出霑). In other words, the period of 霑 is 10 days lasting from the 1st 壬日 after 秋分 to the next 壬日, which is called 霑天.

For example, the 入霑 of this year (2017) is 10/2(壬戌日).



BTW: In traditional calendar, 干支地支 is also used for days, from 甲子 to 癸亥, circulate with the cycle of 60 days.