Going to the house of the second wife on a day or night that is not hers
Question

                Is it permissible for the husband to visit his second wife in a day that is not hers (it is for the first wife) just to ask how her health is because she is ill?.
Praise be to Allah.
    If the husband goes out of the house of the wife whose day it 
    is and goes to the house of the other wife, that is subject to further 
    discussion: 

    (a)

    If it is during the day, and the division is on the basis of 
    nights, then it is permissible in case of need, such as dropping things off, 
    handing over money for maintenance, finding out news, and visiting her if 
    she is sick and so on, because of the hadeeth of ‘Aa’ishah (may Allaah be 
    pleased with her) who said: The Messenger of Allaah  (peace and 
    blessings of Allaah be upon him) did not favour one of us over another with 
    regard to division of his time and how long he stayed with us, and there was 
    rarely a day when he did not go around to all of us, and he would draw close 
    to each wife without having intercourse, until he reached the one whose day 
    it was, then he would stay overnight with her. Sawdah bint Zam’ah said, when 
    she grew old and feared that the Messenger of Allaah  (peace and 
    blessings of Allaah be upon him) would leave her: O Messenger of Allaah, my 
    day is for ‘Aa’ishah; and the Messenger of Allaah  (peace and blessings 
    of Allaah be upon him) accepted that from her. Narrated by Abu Dawood 
    (2135); al-Albaani said in Saheeh Abi Dawood: (it is) hasan saheeh. 

    It is permissible for the husband to be intimate with his 
    wife when he enters upon her, without having intercourse, because ‘Aa’ishah 
    (may Allaah be pleased with her) said: The Messenger of Allaah  (peace 
    and blessings of Allaah be upon him) would enter upon me when it was the day 
    of another of his wives, and he would do everything with me except 
    intercourse. Classed as hasan by al-Albaani in Irwa’ al-Ghaleel 
    (2023). 

    He should not stay with her for long, and if he stays too 
    long he should make it up to the one whose turn it is. 

    (b)

    As for going out to the other wife at night, it is not 
    permissible except in case of necessity, such as if she is gravely ill, or 
    giving birth and her contractions are severe, or there is fear of robbery 
    and fire; if he stays with her too long then he should make it up to the one 
    whose turn it is. 

    Ibn Qudaamah (may Allaah have mercy on him) said: As for 
    entering upon her co-wife when it is her turn: 

    If it is at night, it is not permissible except in case of 
    necessity, such as if she is dying, and he wants to be with her or she wants 
    to give him final requests (a will), or something else that is essential. If 
    he does do that, and he does not stay long before he leaves, he does not 
    have to make it up. If he stays and the sick wife recovers, then he should 
    make it up to the other wife, staying as long as he stayed with (the one who 
    sick). 

    If he goes there for something that is not necessary, he is 
    sinning, and the ruling is that he should make it up, just as if he went 
    there for a necessary reason, because there is no point in making up a short 
    time. 

    As for entering upon a wife during the day when it is the 
    turn of another, that is permissible in the case of need, such as giving her 
    maintenance, visiting her if she is sick, asking about something that he 
    needs to know, or visiting her because he has not seen her for a while and 
    so on, because of the report narrated by ‘Aa’ishah who said: The Messenger 
    of Allaah  (peace and blessings of Allaah be upon him) would enter upon 
    me when it was the day of another of his wives, and he would do everything 
    with me except intercourse. When he enters upon her, he should not have 
    intercourse with her and he should not stay with her for long, because that 
    means that he has dwelt with her and she is not entitled to that. 

    If he stays too long with her, he should make it up (to the 
    other wife). End quote from al-Mughni (7/234). 

    Based on that, if the husband’s going to the house of the 
    wife whose turn it is not happens by day, for the reason that you mention of 
    checking on her, then there is no sin on him for that, and he should not 
    stay there for a long time, except in case of necessity. If it happens at 
    night, it is not permissible for him to go to her unless it is an emergency, 
    such as if she is very sick or the wife whose turn it is gives him 
    permission. If he stays there for a long time then he must make it up to the 
    other wife, spending as long with her as he did with (the wife whose house 
    he went to out of turn). 

    And Allaah knows best.