Australian Citizenship Day is celebrated each year on 17 September.

The importance of this date was introduced in 2001 to provide individuals, schools, community groups and organisations to celebrate and value Australian citizenship and the peaceful, prosperous and inclusive society we share.

There will citizenship ceremonies held across the country, and each new Australian citizens will receive a certificate of Australian citizenship. In this year (2010), Some 3 600 people from 118 countries will become Australian citizens at 69 ceremonies across Australia as part of Australian Citizenship Day.

During the ceremonies, all new Australian citizens are required to sworn to the Australian citizenship pledges, and singing the national anthem. The purpose of the pledge is to ensure newly awarded Australian citizens shall be making a public commitment to Australia and accepting the responsibilities and privileges of Australian citizenship.

The extract of the Australian citizenship pledge is shown below: 

From this time forward, under God, (see NOTE)
I pledge my loyalty to Australia and its people,
whose democratic beliefs I share,
whose rights and liberties I respect, and
whose laws I will uphold and obey.

NOTE: A person may choose whether or not to use the words ‘under God’.

 National Anthem of Australia, "Advance Australia Fair" 

Advance Australia Fair
Australians all let us rejoice,
For we are young and free;
We’ve golden soil and wealth for toil;
Our home is girt by sea;
Our land abounds in nature’s gifts
Of beauty rich and rare;
In history’s page, let every stage
Advance Australia Fair.
In joyful strains then let us sing,
Advance Australia Fair.
Beneath our radiant Southern Cross
We’ll toil with hearts and hands;
To make this Commonwealth of ours
Renowned of all the lands;
For those who’ve come across the seas
We’ve boundless plains to share;
With courage let us all combine
To Advance Australia Fair.
In joyful strains then let us sing,
Advance Australia Fair.

 Sources:

1. 1) http://www.newsroom.immi.gov.au/media_releases/847
2. 2) "Australian Citizenship - Our Common Bond" By Australian Government