‘Daywalker’ was first coined in season nine of South Park, during their oh-so well-known ‘Ginger Kids’ episode.
According to Cartman (and thus adopted by society, obviously), a daywalker is a redhead who can handle being in the sun without stressing too much about getting burnt. They may or may not have freckles or pale skin, and generally have brown eyes.
Like most nicknames for gingers, and despite them having slightly malicious originations, those of us who are proud to have red hair prefer to adopt the terms for the purpose of promoting positivity around ginger hair.
See also: ‘reverse daywalker’ and ‘gingervitis’