I came across these phrases when I read the Elementary volume of the English book Solutions by Oxford University Press, listening 1.30.
In the audio script, there are two sentences: "He usually visits us at the weekend" and "I don't see her during the week, but we often meet at weekends". They confused me since, from these sentences in the speech, they both mean "at many weekends". But obviously, the phrase "at the weekend" is in the singular form and many English sources say that it means "at some particular weekend". I checked both OALD and CALD but they both do not provide any specific explanation about these phrases. Even Britanica discusses somethings about "weekend" but totally misses whether "at the weekend" means "at many weekends" or not (see this: https://www.britannica.com/dictionary/eb/qa/Over-the-weekend-on-the-weekend-at-the-weekend).
Please help to explain these phrases. Thank you.