Unfortunately, there is no set day during the cleanse when this might happen. It may not even happen during the cleanse. It just depends on your body and how it cleanses.
Sorry I couldn't give you a more definitive answer.
Not only is there no set day but passing mucoid plaque on the MC is highly unlikely as after 6 40 dayers and pariticipating on this forum on and off for 6+ years I've only heard of maybe a couple of people claiming to have passed some MP.
Generally after the 3rd day or so it's liquid in, liquid out or as it's come to be known round here, "butt pee"