How the 21-day deadline is actually counted
Day one is the day after the dismissal, and only a deadline that itself lands on a weekend or public holiday moves.
The Fair Work Commission's guidance is specific about the mechanics of the count, and it is worth getting right because a miscount can cost you the deadline entirely. "The 21 days for lodgment does not include the date that the dismissal took effect. This means that day one commences the day following the dismissal."
Weekends and public holidays inside the 21 days do not pause it
This is the detail people most often get wrong. A weekend or a public holiday that falls somewhere inside the 21-day window does not extend the deadline by a single day. The Commission's own words: "Public holidays or weekends that fall during the 21 days will not extend the period of lodgment."
The one situation where the date does move
If the 21st day itself, the final day, falls on a weekend or a public holiday when the Commission is closed, the deadline rolls forward to the next business day. That is the only circumstance in which the date moves; every weekend and holiday that falls earlier in the count is still counted as a normal day.
A worked example
If a dismissal takes effect on a Wednesday, day one is Thursday, and the 21st day falls on the Wednesday three weeks later, whatever weekends or public holidays sit inside that span. If that Wednesday happens to be a public holiday, the deadline moves to the next business day; if it is an ordinary Wednesday, it does not move at all, even though two full weekends and possibly a public holiday fell somewhere inside the count.