TLDR: Foxes do not attack humans. So you can go there day and night. To me this sounds more like a spooking story than reality.: Foxes do not attack humans. So you can go there day and night. To me this sounds more like a spooking story than reality.
Foxes hunt very small animals, humans are way too big for them. Anyway, in their natural habitat they are extremely shy and will run away from you most likely before you are even aware of its presence.
Foxes with rabies can lose their natural fear of humans. I know that Switzerland and UK are considered rabies free and I believe this is true for many European countries. So this is no concern in these areas, in other parts of the world it is a valid concern. Not specific to foxes but in many species.
Another problem is that foxes penetrate urban areas scavenging for food. There they get somewhat accustomed to people. While this mean that the chances of coming across a fox is higher, they are still shy and will never be the aggressor. Of course the usual applies: If you corner the animal or if you threaten pups, they will attack - but only as the very last measure.
To specifically answer the question about whether they attack at day or night: Neither, as explained above. But as they are mainly night active, you are more likely to encounter one at night so the extremely unlikely event of an attack would be more likely at night.
One personal experience: We once grilled chicken in the forest close to a city. There was one fox who tried over and over to get close so it can steal some chicken. Whenever we saw it and made any motion towards it, it ran away. However it was extremely cunning at moving in the shades of the trees, so with time it got quite close, but never reached its goal. :)
