"We are usually at home on the weekend" should be accepted (right now it's not, but I've reported it) since we generally mean more than one weekend when we say "the weekend." To be specific about a particular weekend, you could say, "We'll be home this weekend", or "We were at home that weekend."
*Canadian English speaker here.
The supposedly correct answer given here is: "We use to be at home at the weekends"! That's not a viable English sentence. I would translate this as "We tend to be at home at the weekend" - I know helgerna is the weekends but a direct translation isn't very idiomatic in British English; we would generally say either "We tend to be at home at the weekend" or "... at weekends", so one facet of helgerna is missing in each case. ("... on weekends/on the weekends" may be idiomatic in the US but I'm not sure.) If you've said "tend to be" or "are usually", then "the weekend" implies weekends in general and not just one weekend.