Anicius Aurelius Symmachus was the prefect of the city between 24 December AD 418 and January AD 420. He belonged to the famous family of Anicii and was possibly related to Quintus Aurelius Symmachus. Before his prefecture in Rome, he was proconsul of Africa in AD 415. See PLRE, Aurelius Anicius Symmachus 6.
According to the Liber Pontificalis, Bishop Zosimus died on 26 December AD 418, two days after the installment of the new prefect. Zosimus' funeral took place in the basilica of San Lorenzo fuori la mura; after the obsequies Eulalius and his followers occupied the Lateran Basilica. They were probably waiting till Sunday (29 December) to ordain Eulalius there. The presbyters opposing Eulalius gathered in the church of Theodora (unidentified) and elected the presbyter Boniface as a next bishop. Boniface was later ordained in the church of Marcellus (titulus Marcelli, today's San Marcello in Corso; the present letter is the earliest mention of the church; Liber Pontificalis sets the ordination of Boniface in the basilica of Julius).
The emperor Honorius replied to the letter of Symmachus on 3 January recognizing Eulalius as a valid bishop of Rome and ordering the exile of Boniface (see letter 15 in the Collectio Avellana). Later, however, the party of Boniface obtained a revision of this decision (see the petition of the presbyters to the emperor in [1339]).