There are some other cases where the languages differ about definiteness too, you'll see more examples in the course. But it's probably best just to learn them as exceptions.

Tack så mycket för exemplen och anmärkningar, Arnauti. I didn't see a reply link after your response, so I have replied to my own original posting.

Here are a couple of further thoughts on why the definite article is used with (Eng.) weekend and not with Swedish helg. For me, weekend is not an outlier, but is used with THE exactly like its counterpart, the workweek (working week UK?). I think the reference to etymology is on track, though we can also take a basically synchronic view, that many expressions that use THE (including some proper names, nouns that typically don't need THE) achieve their specificity (and need for THE) through some type of modificational delimitation, whether by means of preposed attribution, a trailing OF phrase, or through noun compounding. The Suez Canal, The Tower of London, the workweek / weekend.

Helg, without the definite article, seems to me to behave more like the words vacation (US), time off, holiday (partic. UK). Unless used to mean a specific holiday, etc., these English words have a more nebulous time frame. They seem more abstract, and abstract nouns in English don't use THE without further modification. Looking at a couple of online etymological dictionaries, one at Project Runeberg, led me to understand that helg, like Eng. holiday, is historically derived from helig dag (holy day) > helgdag > helg. Although etymologically containing a modifier (which in English at least seems lost to contemporary speakers), holy days were numerous, and therefore not specific in the singular. So in Swedish, does helg behave like rast, and lov, as I suspect? What about veckoslut? Does it behave like THE weekend or helg?

With regard to seasons or times of day (as distinct from names of days of the week, which are treated like proper names), you are quite right that THE is more often not used in many such expressions. However, the use of THE is optional there (NOT precluded), and in some expressions obligatory: What do you do in the morning (NOT in morning)? For seasons after IN, I would opt to use THE, though I would not flinch at hearing "in summer." This may be (partly) a US/UK thing, like at (the) table, in (the) hospital, where US usage must have THE.

Finally, as I tried to say originally, THE, though pointing to something specific conceptually (as understood by speaker & listener), can be used not only to indicate specific objects or instances of something, or to indicate a whole group or set (the Smiths, The Bahamas), but also what I regard as a typical or representational example of the whole set or class (and not simply a reference to a particular member of the set): The heart pumps blood. The wolf is an animal. What do you do on the weekend?

våren can be any spring in Swedish, as long as both the speaker and the listener knows which one is referred to, but the difference between Swedish and English when it comes to seasons is that we use the definite in expressions like Vintern är här 'Winter is here' because we consider it to be known what winter we're talking about.

It's it is the weekend that is an outlier in English – you don't say it's the evening, it's the Wednesday today, it's the summer now etc. In another forum, which I can't find now, someone recently suggested that this is because weekend is historically a contraction of 'the end of the week', which makes a lot of sense to me.