We have had two ponds with koi for about the last 12 years. I have noticed
that the main thing that effects feeding is temperature, which can be
related to overcaast days.
Koi cannot digest food when the water temperature drops below 40 or 45
degrees. They can only digest certain types of food at temperatures below
60 or 65 degrees. As temperatures drop, their appetite decreases. Since we
live in northern Nevada, we stock two types of food: one for the spring and
fall, and one for the hot summer. We do not feed the fish at all for the 4
or 5 months of winter.
Perhaps what you are observing as an overcast day, or period, is one where
the sun does not warm the water