Is it considered a sin to say, "Finally, the holiday has arrived, and we've rested from studying," or "I hate studying," knowing that Islamic law has commanded every beneficial knowledge?

The religious texts that mention the virtue of knowledge refer to the knowledge inherited from the prophets, not to worldly sciences. The absolute statement that studying is disliked does not necessitate judging its author as disliking what Allah has revealed, because the dislike is often directed at the system of study, its quantity, and its methodology, not at its subject matter or objective. The person concerned is advised to stay away from such whisperings.