In May my Gross Sales were $26,148
In Dec my Gross Sales were $18,775

By what percentage did my May sales exceed my Dec sales?

gaited - pretty straightforward... almost too easy, really.

To calculate percent change (or by what percent did May sales exceed Dec sales) put May sales in the numerator and Dec Sales in the denominator, then subtract by 1:

$26,148 / $18,775 -1 = .39 or 39%.