When does 秋分后遇壬/逢壬 refer to?
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.