The name-form _Juventius_ for this saint, though preferred by the Diocese of Pavia, derives in all probability from a misreading of the also medievally attested _Inventius_. That in turn is almost certainly a development
from _Iventius_ or _Yventius_, both also medievally well attested for this saint and, more importantly, far closer to the name-form _Eventius_ under which he is recorded as having subscribed both the acts of the synod of Aquileia in 381 and, without indication
of his diocese, the letter to pope Siricius from participants in the synod of Milan in 390. Although one could not learn this from his Wikipedia entry quoted in part by Matt, in modern scholarship he is now customarily referred to as Eventius, following the
form used for him in his earliest documentation. This latter, by the way, is far earlier than anything we have for St. Syrus of Pavia.
Of course, neither the author of the Wikipedia entry nor that of the tendentious and unscholarly one at "Latin Saints of the Orthodox Patriarchate of Rome" on which it is based would find it convenient to mention the
evidence for this saint's having lived in the later fourth century. For that might tend to undermine their dating of both Juventius and Syrus to the first century (modern scholarship prefers the third century for both, though there is some doubt as to whether
Syrus ever existed at all).
By all accounts, none of which is either very early or particularly credible, Syrus (also Sirus) was the first bishop of Pavia. Inventius / Eventius is the traditional third bishop; according to Syrus' eighth- or
ninth-century Vita (BHL 7976), Syrus baptized him and much later, in a vision, predicted the date of his death (8. February, his medieval feast day in Pavia). A tomb in Pavia's basilica dei Santi Gervasio e Protasio, where remains believed to have been those
of Syrus are known to have reposed for centuries prior to their translation to the cathedral, is inscribed SVRVS EPC, i.e. Syrus episcopus:
It is thought that Syrus was laid to rest in this since much rebuilt church and that the inscription (whose EPC portion seems to be a later addition) goes back to a time when he was not yet considered considered a
saint. Herewith some views of his putative relics on display in Pavia's cathedral:
The first we hear of _saint_ Syrus is in a Vita dated to the eighth or the ninth century (BHL 7976) whose purpose is pretty clearly to claim apostolic foundation for the diocese of Pavia and to establish that the latter
was really the original bishopric for the large swath of northern Italy that historically belonged to the archdiocese of Milan. This is done by making Syrus a disciple of St. Hermagoras of Aquileia (who supposedly was consecrated by St. Mark the apostle) and
the apostle not only of Pavia but also of Verona, Brescia, Lodi, and Milan itself, the legendary foundation of whose church by the apostle Barnabas seems to be a response to such posturing. A later medieval Vita and a separate translation account provide other,
sometimes contradictory details but reaffirm the antiquity of Pavia's diocese vis-a-vis that of Milan (of which Pavia was a suffragan from at least the early eighth century onward).
Milan adopted Syrus as a saint of the archdiocese and spread his cult throughout its territory. Further afield, the Syrus venerated at Padua as an early bishop seems in origin to have been Syrus, the protobishop of
Pavia. Opinions differ as to whether that is also the case with the supposedly fourth-century St. Syrus of Genoa.
Some period-pertinent images of Sts. Syrus and Juventius (Eventius) of Pavia:
a) Syrus as portrayed in a relief in Pavia's basilica dei Santi Gervasio e Protasio saved from that church's twelfth-century facade when the latter was replaced in the eighteenth century:
http://ticino.diocesi.pavia.it/pavia/allegati/24051/SAM_1466.JPG
Detail view:
b) Syrus (at right; at left, pope St. Gregory the Great) as depicted in the later twelfth-century frescoes in the crypt of Pavia's chiesa di San Giovanni Domnarum:
https://c1.staticflickr.com/9/8379/8652367226_ea3711f388_b.jpg
There's a detail view in the second row in the _Galleria fotografica_ on this page (image expandable):
http://www.viqueria-storia.com/basilica-cripta-san-giovanni-domnarum-pavia/
c) Juventius as depicted in the later twelfth-century frescoes in the crypt of Pavia's chiesa di San Giovanni Domnarum:
http://tinyurl.com/ookpgw4
d) Syrus (at right; central figure in lower register) as depicted in a fifteenth-century fresco in the chiesa dei Santi Primo e Feliciano (but previously dedicated to San Siro) at Leggiuno (VA) in Lombardy:
http://www.leggiunoturismo.it/wp-content/gallery/sanprimo/9.jpg
e) Syrus as depicted by Vincenzo Foppa in a mid-fifteenth-century panel painting (ca. 1455) in the Minneapolis Institute of Arts:
http://tinyurl.com/alrq2dq
f) Syrus as depicted in the probably later fifteenth-century frescoes (restored in 1984) of the chiesa di San Siro in Lasino (TN) in Trentino-Alto Adige:
http://tinyurl.com/bdoh2n3
g) Syrus (at center) and Juventius (left rear; at rear right, bp. St. Theodore of Pavia) as depicted by Ambrogio da Fossano in a cropped view of a late fifteenth-century panel painting (1491; cropped view) in the Certosa di Pavia:
http://tinyurl.com/q9g88jr
The painting as a whole (smaller image):
http://catalogo.fondazionezeri.unibo.it/foto/80000/58800/58712.jpg
Best,
John Dillon