Using the Evaluate Formula feature may help illustrate that ROW($C$4:$C$14) produces an array {4,5,...14} and ($C$4:$C$14=Find_Date) produces an array of TRUE/FALSE values
The binary for TRUE is 1 and for FALSE is zero so that a ROW number divided by 1 is that ROW and a ROW divided by zero yields #DIV/0
The second argument of AGGREGATE (6) is set to ignore errors so only the numeric values are considered when looking for the Kth smallest. I hope that makes sense.
Let us know if you have any questions.