The rules as they pertain to the UK........
Employees only need a fit note from a doctor after 7 days off work sick and have the right to use their statutory holiday entitlement during their sickness.
Fit notes and proof of sickness
Employees must give their employer a doctor’s ‘fit note’ (formerly ‘sick note’) if they’re off sick for more than 7 days. Hospital doctors or GPs provide them. They can charge a fee if one is asked for before the 7th day.
Employers can also ask employees to fill in a form when they return to work to confirm they’ve been off sick for up to 7 days. This is called ‘self-certification’. Employers usually provide their own version of this form.