How do I derive a text day (Monday, Tuesday,...) from a date field (7/21/2016)

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Better use TEXT(A1;"DDDD") if you eventually want to save in Excel formats, as the "NNN" and "NNNN" format codes are only available in LibreOffice and OpenOffice(.org)

the question was about easier way… not about sophisticated one

Compared to string of IF functions CHOOSE is easier, but of course TEXT is the best and EasyRider should choose that as the best answer. I looked for FORMAT and didn't realize that function was called TEXT.

And your answer is wrong, also if it produce accidentally the "right" results.
do never patch some Date-formatting on Values which are no real Dates

The main formula I recommended is =Text(A,"DDDD"), though it wrongly works with Weekday function.