I have observed in the 1st series of 'The Haunting of Hill House' on Netflix that all the members use to come back into hill house at night time only. Why?
The OP seems to be asking why all the family members only returned to the house at night when they were grown-ups.
Nell was urged by her therapist to return and confront the house. She puts this off out of fear, but eventually she does go. In the show she acknowledges that she shouldn't have waited all day until it was dark, but she doesn't have any reason not to go at night other than it being scarier than the day. So she convinces herself to go to confront the house.
Luke decides to return to the house so he can burn it down. He doesn't decide this until after the funeral of Nell which took the whole day. His decision is made abruptly and passionately and he didn't have any reason not to go at night as opposed to the day. He was simply reacting to the moment.
Steven, Shirley, and Theo
The rest of the siblings go at night because they are going to stop Luke. Luke went at night so the rest of the siblings went at night.
Why not at night?
Although the question doesn't specify, I'm assuming the OP thinks they should have known not to go at night since the show establishes that the house comes alive at night. However, none of the children knew this. This fact is revealed in a conversation between Mrs. Dudley and the mother and the kids had no reason to know that the house should be avoided specifically at night.
Like Luciano explained, it's a common horror story/haunted house trope, which is used to help generate suspense and create symbolism.
However, and although this doesn't exactly answer your question in direct sense, but in an indirect way because of what it does for the story, the story establishes some mythology that Hill House (and it's inhabitants) tends to come more alive at night, as explained by Mrs. Dudley, as it's one reason she and her husband do not work there at night.
That's not to say that we didn't see occurrences in the day time either, but as the story progresses the viewers come to understand that Hill House
is really a kind of space-time vortex where the house wants to eternally keep people in state of happiness post death. As Nell explains, the red room is the heart and it "created" rooms for each of the children to happy isolated from each other and that time is not linear, but rather it's like snow or confetti, in which most instances that the children experienced that were scary was really themselves through out space and time! However there are also hints at "possession" since I think we didn't really understand the full back stories of the Hills in relation to what happened to Olivia and/or if Poppy's philosophy about the house being good and the outside world being being viscous and cruel is true or if she is truly crazy? There are a lot of arguments >! one could make on both sides.
But the fact that so much of what occurs happens at night, including the current scenes in the final episode, makes it an important mythological point, but more over the symbolism of a philosophical debates about what is happiness in relation to life vs death, togetherness vs isolation, the family you have or the family you make, or if adulthood is the end of happiness?? Is there light in the dark or dark in the light or which is better??
I'm not sure the story answers any of these, since individual or interpersonal stories end in different places for different characters and there are some loose threads with a possibility of the second season...