Swap first and second columns in a CSV file

Here you go:

awk -F, '{ print $2 "," $1 }' sampleData.csv 

Because default delimiter is space/tab. Not comma. Taking your original code, and adding a -F, solves it.

$ awk -F, ' { t = $1; $1 = $2; $2 = t; print; } ' /tmp/2016_logins.csv
day num_logins
2016-07-01  253
2016-07-02  127
$

Stripping it down to {$0=$2" "$1}1 gets same result.

$ awk -F, '{$0=$2" "$1}1' /tmp/2016_logins.csv
day num_logins
2016-07-01  253
2016-07-02  127
$