I didn't mean to imply that it must be plural, just pointing out context that makes plural possible. Singular and plural are equally correct because "etek" is a general/unspecific direct object.
For the same reason, your example of "Her gün takım elbise giyer" could also be "He wears suits every day."
You can look at it either way because we only wear one suit or skirt at a time, but presumably we're not wearing the same one every day (or every Sunday). Both perspectives (and both translations) are valid. :-)
More info on singular/plural direct objects: here
In Turkish "etek" is ambiguous. It can mean "skirts" or "a skirt". You have to say "bir etek" if you want to indicate only the singular. In English "I wear skirts on Sundays." and "I wear a skirt on Sundays." have the same meaning as a general statement of habit.
I expect the plural makes sense in English because Sundays is plural and one would not wear the same skirt every Sunday.