Where "Mufti Day" Comes From
Worth knowing the fuller history here. "Mufti" is originally an Arabic word for an Islamic legal scholar. English borrowed it by 1816 to mean "civilian clothes" in a military context — the 1863 glossary Hobson-Jobson suggests it came from British officers in colonial South Asia jokingly comparing their own off-duty dressing gowns and slippers to the traditional dress of a mufti. Some writers have pointed out that this makes the term's origin an act of British colonial troops mocking the clothing of the people they were then occupying.
That colonial military sense of "civilian clothes, not uniform" is what carried through into Australian schools, where "mufti day" (a day students wear their own clothes instead of uniform) is documented from around the 1960s. The word's modern use has nothing to do with its Arabic root or its colonial history — but the fuller history is worth knowing.
Source:
"Don't forget it's mufti day tomorrow, no uniform needed."
How Aussies Actually Use It
It's used almost exclusively in a school context, usually announced ahead of a themed dress-up or fundraiser day.
Students often pay a gold coin donation to take part.
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