|Observed by||Empire of Austenasia|
|Significance||Anniversary of the recognition of the imperial rank of the Austenasian Monarchy|
|Celebrations||The granting of honours and the creation of imperial foundations by the Monarch|
Imperium Day is a public holiday in the Empire of Austenasia observed on 4 June, which celebrates the imperial rank of the Austenasian Monarchy and the Roman heritage and culture of the country as a whole.
The holiday was established on 14 January 2014 by the Public Holidays Act 2014, and first celebrated later that year. It falls the anniversary of the recognition of the imperial rank of the Austenasian Throne by the then Emperor Sebastian I of the briefly revived German Empire in 2011.
Emperor Jonathan I has used the holiday as an occasion for imperial foundations, creating the Imperial Library in 2015 and the Imperial Numismatic Museum in 2018, both in Wrythe. The Emperor has used each Imperium Day since as an occasion to donate coins of emperors to the Imperial Numismatic Museum. The holiday has also seen the granting of honours, with the Emperor making appointments to his Privy Council in 2015, and in 2017 publishing an honours list of the size usually reserved for Independence Day; a smaller honours list was also published in 2019.
The holiday has been celebrated nine times since its creation; however, three of these occasions have been marred by other events. In 2014, the holiday took place immediately after the invasion of New South Scotland by Zealandia; in 2016, it coincided with the resignation of Countess Eritoshi from politics; and in 2017, it took place the day after a terrorist attack in London, mere miles from Wrythe.