Max's Home, Early Evening
A hallway, a staircase, a bedroom. This story could take place in anyone's home anywhere in the world, during any time period. There's nothing to anchor it to one particular country, culture, or time (except maybe the fork).
The moon showing through the window, the color of the sky, and the dimness of Sendak's illustrations do show us that the story takes place in the evening. And we know that Max's mom is in the middle of making dinner, so we can guess that the action takes place during what many parents refer to as "the witching hour": that time of day just before dinner when the kids are tired and hungry and especially prone to acting out.
Still, even with the twilight designation (not that Twilight), Where the Wild Things Are is a universal story of childhood, so it's appropriate that a specific location or time for the story is never identified. href="https:>
Sure, a forest grows in Max's room and he travels to the land of wild things, but in truth, we know that he takes this journey in his imagination. Which means all the action of the story takes place in his home, a home that could be almost anywhere in the world in any era (as long as there are forks).