Explanation
Working from the inside-out, the WEEKDAY function takes a date and returns a number between 1 and 7. With default settings, the number 1 corresponds to Sunday and the number 7 corresponds to Saturday.
The CHOOSE function simply maps numbers to values. The first argument is the number to map, and subsequent arguments represent associated values.
In this case, 7 values have been provided in the order required to work with WEEKDAY's Sunday through Saturday scheme.
With a date from column B, WEEKDAY returns a number which is fed to the CHOOSE function. CHOOSE returns the value at that position in the list of abbreviations.
Related formulas
Get day name from date
Related functions
WEEKDAY Function
The Excel WEEKDAY function takes a date and returns a number between 1-7 representing the day of week. By default, WEEKDAY returns 1 for Sunday and 7 for Saturday, but this is configurable. You can use the WEEKDAY function inside other formulas to check the day of week.
CHOOSE Function
The Excel CHOOSE function returns a value from a list using a given position or index. For example, =CHOOSE(2,"red","blue","green") returns "blue", since blue is the 2nd value listed after the index number. The values provided to CHOOSE can include references.
