https://www.awm.gov.au/collection/C1106639?fbclid=IwAR29ZvFLkXIuqqAglBjIz8svS2yxSOBykIUUNN6nB1HS5-PjhSMWgwFm6HI

This link highlights the original Australia Day which was held in 1915.

It was decided to then be held on the 4th Friday on July every year.