SUM(...)

Category: Math and trigonometry

Adds its arguments

SUM(number1,[number2],...)

Category: Text

Formats a number and converts it to text

TEXT(Value you want to format, “Format you want to apply to that value”)

Category: Lookup and reference

Returns the row number of a reference

ROW([reference])

Category: Lookup and reference

Returns a reference indicated by a text value

INDIRECT(ref_text, [a1])

1

The formula takes the difference of two dates and adds it up in the start date to get all the dates in the range. Then it uses TEXT() function to format it as first three letters of the day and then perform match and sum the count.