Why were the Jewish believers still practicing the seven Jewish feast days at the time of Pentecost?

At that time no one had told the Jewish believers or anyone else to cast aside Temple worship or the Law. The Temple was still operating, so they continued observing the seven Jewish feast days. Pentecost itself was the fiftieth day after Passover, and they were waiting in Jerusalem as Scripture records.

Les explains the setting in Acts 2: Christ had just been crucified at Passover, then He was with them 40 days in His post-resurrection ministry, and they waited another ten days until the fiftieth day, Pentecost. Every one of the seven Jewish feast days brought Jews to Jerusalem from the whole then-known world. The Temple was still operating, and no one had told these Jewish believers or anyone else to cast aside Temple worship or the Law. So these Jews were still practicing the seven feast days when Pentecost came. Scripture then points out that when the day of Pentecost was fully come, they were all with one accord in one place, and suddenly a sound from heaven like a rushing mighty wind filled the house where they were sitting.

Were the Jewish believers still observing the seven Jewish feasts when Pentecost arrived?

Yes. At Pentecost, Jewish believers were still observing the seven Jewish feast days. The Temple was still operating, and no one had told them to cast aside Temple worship or the Law. Scripture presents Pentecost as the fiftieth day after Passover, with these Jews still practicing those feast days.