if you say you will do something come rain or shine, you mean you will do it whatever happens.
Come rain or shine, I'll be there, I promise.
come rain or shine
No matter what the weather is. Pack a poncho because the outdoor concert is happening come rain or shine.
come rain or shine
and come rain or (come) shine
no matter whether it rains or the sun shines; in any sort of weather. (See also rain or shine.) Don't worry. I'll be there come rain or shine.We'll hold the picnic—rain or shine.