While there is no legal requirement to use your bike light during the day, there are a few exceptions to this rule. Riding a bike in “limited visibility conditions” without a light is considered to be an offence, as well as being very dangerous to yourself. Hence why it’s a good idea to keep your tail light to hand at all times, should you find yourself in this situation. Weather that constitutes “limited visibility” includes heavy rain, fog, smog and so on, so if you know this is likely to happen, definitely bring your bicycle tail light along.
As well as being a legal requirement, it’s always a good idea to have a bike light with enough lumens to be visible during the day. Whether you cycle along busy roads and are working your way through rush hour traffic every day, or find yourself along a quiet road, it’s always worth making yourself a little more visible to other vehicle users.