Australia

I'm with you. 3rd March is a good pick for both what it represents (throwing off the shackles of the British) and for the time of year (early Autumn is basically still summer), and I strongly support the idea of changing it to that date.

I think I would prefer 3rd September or 9th October, the date the Statute of Westminster was enacted (depending on whether you go by the date it actually passed in 1942, or the date it was retroactively applied to in 1939). They both come at a time of year where there's a relative lack of public holidays. The 5 month period from late December to early May already have Christmas, Boxing Day, New Year's, Good Friday, Easter Monday, ANZAC Day, and Labour Day. That's 7. The entire rest of the year has just 2!

(Using Queensland dates here, but the trend is similar elsewhere, with a lot of state or local public holidays also being in that span. And, obviously, I'm discounting Australia Day from the 5-month period for the sake of the hypothetical. Otherwise it would be 8.)

Ideal would be an early November/late October holiday, to get that warm weather while still being reasonably removed from the Christmas–Labour Day period. The early October of the date the Statute of Westminster Adoption Act received royal assent is a little earlier than ideal, but has the added bonus of some synergy with ACT, NSW, and SA's Labour Day, as well as Qld's King's Birthday, giving two long weekends in a row: sometimes (including this year) a 3-day week, usually two 4-day weeks in a row.

But any of these dates would make great alternatives and have my strong support.

3rd March 1986 is the date the Australia Acts were enacted, which is actually what removed the last shackles of UK dominion over Australia, including the UK's ability to pass legislation that would have effect in Australia or for Australians to appeal to the UK privy council. Up until this point, Australia was officially still a dominion of the British Empire.

The Statute of Westminster was officially adopted on 9th October 1942, though it was backdated to 3rd September 1939. It removed most of the ability of the UK to legislate with effect in Australia (still enabling them to legislate over Australia with Australia's "express request and consent"), removed the ability of the monarch to refuse to allow the Governor-General to give royal assent, and gave Australia the ability to control the laws of royal succession independently of the UK, which is why prior to William having his first kid, Australia (and other countries such as NZ, in addition to the UK itself) had to enact legal changes to the laws of succession in order to allow for succession to be simple primogeniture, rather than male-preference primogeniture.

So that's 3 good dates there: 3 March, 3 September, and 9 October.

There's also 9 July, which is the date the Australia Constitution Act 1900 was enacted by the UK Parliament, allowing for federation to occur on 1 January the following year. So a potential 4th date, but kinda a lame one IMO.