I think lonesomeday has a good answer, but I would like to add another dimension:
The festivals and sacrifices being observed in Israel at the time of Amos were also displeasing because they were bound up with idol worship and violated many of the statutes God has given regarding them. The vision Amos receives is written to the people in the day of Jeroboam son of Jehoash (Amos 1:1) when they were going to Bethel to make sacrifices (Amos 3:14, 4:4, 5:5-6).
The sins of Jeroboam son of Jehoash follow the pattern of the kings of the northern kingdom: "He did evil in the eyes of the Lord and did not turn away from any of the sins of Jeroboam son of Nebat." Jeroboam son of Nebat set up the worship system in the northern kingdom that persisted to the day of Amos and Jeroboam son of Jehoash, and which we ready about in 1 Kings 12:28-33 (ESV):
So the king took counsel and made two calves of gold. And he said to the people, "You have gone up to Jerusalem long enough. Behold your gods, O Israel, who brought you up out of the land of Egypt." And he set one in Bethel, and the other he put in Dan. Then this thing became a sin, for the people went as far as Dan to be before one. He also made temples on high places and appointed priests from among all the people, who were not of the Levites. And Jeroboam appointed a feast on the fifteenth day of the eighth month like the feast that was in Judah, and he offered sacrifices on the altar. So he did in Bethel, sacrificing to the calves that he made. And he placed in Bethel the priests of the high places that he had made. He went up to the altar that he had made in Bethel on the fifteenth day in the eighth month, in the month that he had devised from his own heart. And he instituted a feast for the people of Israel and went up to the altar to make offerings.
Besides the idolatry, there are couple things worth pointing out:
- God had commanded that the people worship him at the temple in Jerusalem (Deut. 12:11-14). They were not permitted to worship at Bethel.
- God had commanded that only the people from the tribe of Levi were permitted to serve as priests. But the priests at Bethel were from any tribe.
- God had commanded that the festivals be observed on certain days (Lev. 16:29), but Jeroboam had selected other days and month "in his own heart."
The whole situation sadly recalls what happened in Exodus 32:4f, where even as the Law was being given the people had Aaron fashion a calf of gold who then proclaimed "Here are your gods" (cf. 1 Kings 12:28) and ordained a festival and sacrifices. The people then, too, indulged in revelery not too dissimilar from the sins of Israel mentioned in Amos 2:7-8. God was not pleased with the people in Exodus 32; so neither was he pleased with them in the days of Amos when Jeroboam led the people into sin in this way.