Just as a side note, remember that it's "on weekends" in AmE, but "at weekends" in BrE.
These exams are even held at weekends!
I went to visit my parents at the weekend (last weekend).
I'm going to visit my sister at the weekend (next weekend).
There is one example I can think of where "the weekend" is used, but doesn't refer to a specific weekend:
My friends and I usually go out at the weekend.
No doubt, this should be said "...usually go out at weekends", but in BrE at least, saying "usually + the weekend" it's understood that we mean "most weekends".
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