“the day of the week” meaning just one day. for example:sunday is the first day of the week. monday is the one day of week. "the days of the week" meaning several days. for example:The English names of the days of the week come from Latin. We've learned about the names of the months; let's look at the names of the days of the week: Sunday,Monday,Tuesday,...Saturday.

to put it simply the day of the week = the full name of the day of the week however ,the days of the week = the seven days of the week can u understan me ? O(∩_∩)O~