Western Australia marks King's Birthday on the last Monday of September, months after every other state observes the same public holiday name. 12 September 2027 carries this public holiday only in Western Australia, with the rest of the country working through an ordinary day under a completely different holiday calendar for the same occasion.
Any deadline or bill payment due on Sunday 12 September 2027 moves to the following Monday under standard business-day rules, since Sunday sits outside the working week this page counts by default. This convention applies consistently across contracts, invoices and statutory deadlines throughout Australia, regardless of which state or territory is involved.
2027 has 365 days on the calendar. Leap years add a single extra day to February, giving those years 366 days instead of 365 and shifting the day-of-year number for every date from March onward by one compared with an ordinary year. This is the only irregularity in the length of the Australian calendar year.