On the twenty-fifth

By 1758, Frederick of Prussia was struggling to defend his kingdom against three large allied powers, Austria, France, and Russia. In August that year, a Russian army of forty-four thousand men under General Fermor crossed the Prussian frontier and, on the twenty-fifth, fought a battle with Frederick and thirty-seven thousand Prussians near the town of Zorndorf. The nine-hour battle was among the bloodiest of the eighteenth century:

Could anyone explain the meaning of ' on No. 25' ? what does the number refer to? Thanks!