Julian (Flavius Claudius Iulianus Augustus), known as Julian the Apostate for his rejection of Christianity or Julian the Defender to modern Pagans for his defense of Paganism, was the last Pagan emperor of Rome, from 355 until his death in 363. He attempted to restore the status Hellenic Paganism had enjoyed in the Roman Empire before Constantine I had made Christianity the state religion earlier in the 4th century and guaranteed freedom of religion.
He might have succeeded in consolidating his victories into a more lasting legacy, but on June 26, 363, while on a military campaign against the Persians, Julian was killed during a skirmish at what is now Samsara, Salah al Din governorate (state or province), Iraq, and the Christians returned to power in Rome, eventually making martyrs of untold millions of Pagans and adherents of other religions and even forcing destruction on Earth itself.
Yet Julian remains as a shining example of positive Pagan virtue, one with respect for the past and hope for the future.
One may honor him and his legacy on June 26, the anniversary of his tragic and untimely death.
One way to do this is suggested here: http://www.juliansociety.org/meditation.html
Other links about Emperor Julian:
Julian Society page about his life and legacy
Hellenion voted in 2018 to add the anniversary of the death of The Roman Emperor Julian to our official calendar.