There is no such EU law. Post is not delivered 5 days a week in Sweden.
- a service (collection from access points and delivery) on 5 working days a week (with exceptions);
- the collection, sorting, transport and delivery of postal items weighing up to 10 kg;
- services for registered and insured items." [1]
That being said, obviously the specific number of days is not important here.
[1] https://eur-lex.europa.eu/EN/legal-content/summary/postal-se...