Serbia waited almost the full 48 hours allowed to them (until 6PM on July 25) to respond to Austria’s ultimatum, but others responded sooner:
Russia, who considered the Orthodox Serbs their brothers (a feeling reciprocated by the Serbs), “would not tolerate another abandonment of their brothers, the South Slavs. Russia would be disgraced, would have no more friends in the Balkans, no respect in Europe. A failure of such magnitude might trigger a revolution. . . . One solution suggested itself. If Russia showed enough firmness, perhaps Austria would hold back.”
~G.J. Meyer, A World Undone: The Story of the Great War, 1914-1918 (New York: Delacorte Press, 2006), 38.
In consequence, it was on the 24th (the day before the Serbian response to the Austrian ultimatum was delivered) that the first small step towards war was taken . . . the Russian foreign minister Sergei Sazanov advised the Russian army’s chief of staff to prepare for mobilization.