Answer
The specific date of Étienne Aubert's appointment as Cardinal-Bishop of Ostia and Velletri is not provided in the text. However, it is understood that he held this position during the period leading up to his pontificate as Pope Innocent VI, which began in December 1352. Therefore, it can be concluded that Étienne Aubert's tenure as Cardinal-Bishop of Ostia and Velletri occurred sometime before his ascension to the papacy.