A calendar day is not a working day
Ten days of work do not usually end ten calendar days after the project starts.
In a standard Monday-to-Friday schedule, weekends fall inside the delivery window. A weekday public holiday pushes the finish date further.
A public holiday does not increase effort
A holiday does not add work to the task. If the task needs seven working days, it still needs seven working days.
The non-working day only moves the point at which the seventh day of work can happen. This is why effort and calendar duration should be planned separately.
Use the calendar that controls delivery
The team may be in Poland, the client in the United Kingdom and another contributor in Germany. Their public holidays do not always fall on the same dates.
When one person or team owns the work, start with their calendar. If delivery depends on several parties, make those dependencies explicit instead of hiding them in one date.
Check regional differences
Some public holidays vary by region. A planning calculator is useful for operational estimates, but legal, contractual or regulated deadlines should always be checked against the rules that apply to the specific location and agreement.
The difference grows with time
For a two-day task, the gap between working days and calendar days may be small. Over several weeks, weekends, holidays, leave and dependencies become much more likely to affect delivery.
The longer the horizon, the less reliable manual date addition becomes. Use the Delivery Date Calculator and review the complete delivery-date method before communicating a commitment.