British English and American English often choose different prepositions with the same time expression. One common example is at the weekend and on the weekend.
In standard British English, people usually say at the weekend. In standard American English, people usually say on the weekend. The meaning is the same, so this is mainly a difference in regional preference, not a difference in meaning.
- British English: We are visiting friends at the weekend.
- American English: We are visiting friends on the weekend.
You may also hear on weekends in American English and at weekends in British English when people talk about habits or repeated actions.
- American English: I usually relax on weekends.
- British English: I usually relax at weekends.
If you are learning English, the best choice is to stay consistent with the variety you are using. If your English is mainly British, choose at the weekend. If it is mainly American, choose on the weekend.