* I chose to use a comma for a two-days range, so Mo,Tu rather than Mo-Tu
* This always starts on Monday, i.e. it won't do Fr,Sa,Su,Mo => Fr-Mo.
Maybe for the USA it would make more sense to start on a Sunday.
Or maybe we want to start from the last day that was set, starting Mo,
so the above would lead to Fr-Mo.
Shorter, but less readable than Mo,Fr-Su, arguably?