weekdays() is locale-sensitive, should only be used for formatting, not for calculations

[thiloplanz]

The README file recommends:

For this part the weekdays() function may be of some help here.

This function returns strings that are locale-sensitive, though.

If one were to then use pattern matching to determine if a given day falls on the weekend, that code may not work on another person's machine (where the names are from a different language).

Maybe strftime (which can return a 0 - 6 number for the weekday) is a better choice, especially in a class about reproducible code.

[thiloplanz]

An alternative is to keep using weekdays, but explicitly tell R to use an English locale.

This is done for example at https://github.com/Gianfranco-Campana/RepData_PeerAssessment1/blob/228f91625f6efd9722965ac4aa08e980f91ca2e5/PA1_template.md#change-my-locale-to-obtain-english-week-names-rather-then-italian-then-transform-date-in-posixt-format

  Sys.setlocale(category = "LC_ALL", locale = "C")