I have code that will ask for the day of the week but is there a way to make it ask the question again if the user doesn't enter one of the 7 days provided?
cout<<"Please provide a day of the week - ex. Mo Tu We Th Fr Sa Su"<<endl;
transform(day.begin(), day.end(), day.begin(), tolower);
Well let's look at what you want to do logically.  prompt user to provide day of week  read user input  check user input for validity + if user input is valid; do what you need to do - if user input is invalid; prompt user about incorrect input and ask again
Since you don't know how many times the user may enter incorrect input you could use a do while loop to check for validity. That is logically, while(input is false/invalid) prompt user for new input. You will need a flag of type bool.
bool inputValid; // should be true if input is accepted
cout << "That isn't a day of the week!\n" << "Please provide a day of the week - ex. Mo Tu We Th Fr Sa Su" << endl;
cin >> whichday;
// you'll need code in the if statement to check whichday against accepted input
if (/*whichday is accepted then*/) inputValid = true;
else /*whichday is not accepted*/ inputValid = false;
while(!inputValid); // while input is not valid