Both the first and last days of 2019 were a Tuesday.

This means that index 0 is Tuesday. Consequently:

0 is Tuesday

1 is Wednesday

2 is Thursday

3 is Friday

Youâ€™re also supposed to reverse the order:

Create a function, best_friday , that takes list_of_customers as input and returns the best customers on each Friday, ordered from the last Friday of 2019 to the the first one.

So your solution, in a sense, is incorrect in two different ways.

The first (01/01/19) and last day (12/31/19) of the year are both Tuesday so the elements at index 0 and -1 are Tuesday. I also got confused because I completely forgot Tuesday was also the last day.