at the weekend British English on the weekend American English
I never work at the weekend.
What are you doing on the weekend?

at weekends British English on weekends American English
I only see him at weekends.
Tony has been unwell over the weekend (=during the weekend).

The expressions are interchangeable. The weekend is the abstract noun; weekends is the concrete nouns in the plural. This is a subtle point, and shouldn't cause any worry.