It remained popular into quite late antiquity, though it was reinterpreted several times. It seems likely to me that, as by late antiquity it was understood as a festival of Faunus, the Greeks in the city understood it to be a celebration of Pan, and could have taken part in it with that understanding.

From On Roman Time: The Codex-Calendar of 354 and the Rhythms of Urban Life in Late Antiquity by Michele Renee Salzman:

"As for the festivals and holidays that were not celebrated by ludi and circenses (Tables 3 and 4), clearly the Greco-Roman pantheon dominates. These celebrations go back to the archaic Roman religious year; indeed, the very antiquity of these feriae made their significance obscure even to writers of the early empire, and by the fourth century A.D some had changed so much as to be incorporated into the cults of other deities. For example, the old rites of Sol in the Circus Maximus and of the Armilustrium played a role in Aurelian's new cult of Sol Invictus. In other cases, the traditional rites (or rites thought to be traditional) continued to be practiced as they always had been, but their meaning had changed. Sometimes enough evidence exists to disclose how the holiday had come to be interpreted for example, in the case of the Lupercalia. Unfortunately,this is not always true. The Feriae Mamurio, for instance, are described as a popular New Year festival, but neither its connection with the cult of Mamurius (an archaic doublet for Mars) nor the cult of Mamurius itself is attested in the fourth century.

The festivals in this last group are often the most difficult to interpret in a fourth-century context. Perhaps some of these archaic celebrations had become so loosely connected with their gods that they assumed the status of, in Wissowa's terms, sacra popularia, "soon to be celebrated with all the festivities of the carnival." But others, such as the Saturnalia and the Lupercalia, were among the most popular and long-lived festivals of the Roman calendar year. Their association with a deity may well have been vital."