When a shipping estimate, contract, or deadline says 'within 5 days,' it matters enormously whether that means 5 calendar days (every day counted, weekends included) or 5 business days (weekends and holidays skipped). The two can differ by several actual calendar days depending on when the count starts.
Where each convention is typically used
Shipping and delivery estimates, legal notice periods, and most business/contractual deadlines conventionally use business days unless stated otherwise, since they're describing time during which business operations (processing, fulfillment, review) actually happen. Simple everyday deadlines ('due in 3 days') more often mean calendar days, since there's no implied dependency on business operations.
The starting-day ambiguity
A second common source of confusion: does the day the count starts (day zero) count as day one, or does counting begin the following day? Different institutions handle this differently, and it's often not stated explicitly. When a deadline genuinely matters, it's worth confirming both which day-type convention and which starting-day convention are being used, rather than assuming.
Calculating either one correctly
The business days calculator counts forward or backward from a given date while skipping weekends (and can account for public holidays), so you can get an exact target date instead of manually counting on a calendar and risking an off-by-one error around a weekend.