Day Walking is an ability that allows vampires to walk in sunlight without being burned. Users who have this ability are generally referred to as "Day-Walkers".
Original vampires and non-Original vampires can't walk in daylight without being burned, but they can survive in daylight with Lapis Lazuli which is a stone that witches have been able to enchant to allow the Original vampires and non-Original vampires to walk in the daylight, the stones are usually fixed onto a ring, amulets and bracelets. Without the Lapis Lazuli item, the Original vampires will burn and non-Original vampires will simply burn to death.
Depending on the amount of blood drank, vampires grow immune to their weakness to sunlight after drinking fairy blood. The effects last shortly, however, and the pureness of the blood is a factor too.
When a vampire is given the blood of Alexander Corvinus is able to withstand sunlight.