Calendar days vs business days
Calendar days count every day including weekends and public holidays. Business days count only Monday to Friday, excluding public holidays. Whether you need calendar or business days depends on context — contract deadlines and legal timelines often specify business days; project durations and countdowns usually use calendar days.
In Australia, "business days" in a legal or contractual context typically excludes both weekends and national public holidays. State-specific holidays may also be excluded depending on the state in which the contract is enforceable.