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.

Les teaches that the setting of Acts 2 is crucial. Christ had just been crucified at Passover, and the believers were waiting for Pentecost, the fiftieth day. After His post-resurrection ministry of forty days, they waited another ten days, bringing them to the fiftieth day. Les explains that every one of the seven Jewish feast days brought Jews to Jerusalem from the whole then-known world, from Great Britain to India, from Northern Africa across the Mediterranean to Greece and Asia Minor. He emphasizes that the Temple was still operating and that no one had told these Jewish believers or anyone else to cast aside Temple worship or the Law. Therefore, these Jews were still practicing the seven feast days when Pentecost arrived. Les then points to verse 1, where the day of Pentecost was fully come, the fiftieth day after Passover, and they were all with one accord in one place, followed by the sound from heaven as a rushing mighty wind filling the house where they were sitting.

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.