What is the ruling on entering the co-wife's house on her designated night or day?

The correct ruling regarding the prohibition of entering upon a woman who is not the wife of the night, except out of necessity at night, or for a need during the day, is to refer to the prevailing custom and the common practice of people. So, if his entering upon another woman, whether at night or during the day, is not considered an act of injustice or wrongdoing, then it is permissible. This is because referring to custom is a major principle in matters for which there is no specific religious text.