What does "the day of the week" and "the days of the week" mean? Is there any different? Thanks before :)

Question

Answer

“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.

Answer

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~