Traditionally, today is the Vigil of All Saints, a feast introduced to Christianize the celtic festival of Samhain. Unofficially it recalls the souls of the damned, thus serving as a reminder of hell and need to avoid it. The Vigil was abolished however in 1955.
Some traddies are nervous about Halloween practices, seeing them as pagan, but I can't see why, it was adopted in Christian countries long ago. All the same, Halloween isn't really a traditionally Australian practice, but an American custom that has spread. Still, a fun one for the kids that has a useful underlying message so why not!
Today is traditionally one of fasting. But there are traditional goodies and candies associated with the evening which you can read up on over at fisheaters.com (note however that its advice on indulgences for All Souls is incorrect, not being consistent with the current Handbook of Indulgences).
Preparing for All Saints and All Souls
And today, consistent with the purpose of a vigil, is also a chance to get ready for All Saints and All Souls. In most of the world, All Saints (Saturday) is a holy day of obligation - not however Australia. Still, plan on going (and of course if you are in reach of Sydney, don't forget the Solemn Mass at St Anne's Bondi tomorrow)!
The most important thing is to get to confession so you can collect an indulgence for the souls in purgatory. Basically the deal is that on All Souls Day, provided you have been to confession (or get there within the next week) and received communion, you can gain a plenary indulgence for the Holy Souls by visiting a church or public oratory, making the usual prayers for the Holy Father, and reciting the Our Father and the Creed. Just remember that you need to be free from all attachment to sin at the time you say the prayers (this means attachment to any particular sins, including venial ones) or the indulgence will only be partial.
The slightly strange calendar situation this year also offers some particular gifts. There is the possibility of celebrating two All Souls Days - on Sunday with the Novus Ordo, and again on Monday in the Extraordinary Form. And priests (and those who want to attend) who are bi-ritual can potentially celebrate the three masses of All Souls on Sunday in the Novus Ordo - then again on Monday in the Extraordinary Form!