I let mine out during the day, but never at night as that is the time when most accidents occur to cats, whether traffic or predators. Thank goodness there are not many of those in the UK, though I did lose a kitten once to a fox.
Usually I let them out around 6 months of age, after fixing. I start them on a lead, to get them used to the near surroundings, and then with me but without the lead, making sure I have a treat for them. And by then I have trained them to (usually!) come when they are called, especially if it means a treat. Gradually I can let them explore further on their own, once I am sure they know the ways back home. Occasionally I have a panic when one is not in by dusk, but none of them is actually nocturnal now, they all like to sleep at night!