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])

0

The formula checks for each date if it is Sunday using TEXT() function and then return and array of zero and ones that is summed up to give you the answer