Austria sent the ultimatum to Serbia on July 23rd 1914
Serbia
When Serbia responded to Austria-Hungary's ultimatum, they accepted all the conditions save one. Austria-Hungary immediately declared war on Serbia, which had always been their intention given the demeaning demands they had made.
23rd July 1914
Because the the Archduke Franze Ferdinand was the heir to the Austria-Hungary throne when him and his wife when assassinated by a Serbian Nationalist in Sarejevo. Austria Hungry Gave Serbia a ultimatum and when Serbia did not follow the ultimatum Austria-Hungry declared war with Serbia
I am guessing that this question is referring to the Ultimatum given by the Austria-Hungarian government to Serbia just before WW1. The whole point of the conflict between Serbia and Austria at the time was a conflict between Russia and Austra fighting for allegiance of the strongest country in the Balkans, and Serbia feeling threatend in its sovereignty. The ultimatum was in a form which was apparent to everyone that Serbia could not accept, since the political consequences of agreeing to it and loosing a war were the same.
Germany could have refused to back Austria on the grounds that the ultimatum to Serbia was too harsh. ;)
when Serbia refused to comply with Austria- Hungary's ultimatum
Austria was rightly angered. They issued an ultimatum to Serbia to right the wrongs committed, but the ultimatum was too much for just an assassination. Serbia didn't reply and Russia stepped in to help the small Slavic brother. That move turned the two-nation conflict into a war.
The assassination of Archduke Franz Ferdinand of Austria, by Gavrilo Princip on June 28, 1914. On July 23, 1914, Austria set an ultimatum for Serbia to leave Austria. This ultimatum would expire on July 28, 1914. When the Serbs would still not leave, Austria declared war on Serbia.
The Archduke of Austria Franz Ferdinand was assassinated by a Bosnian-Serb. In retaliation Austria-Hungary gave Serbia an Ultimatum of demands designed to destroy Serbia's influence in Bosnia. Serbia rejected the Ultimatum and Austria-Hungary declared war on Serbia. Germany was Austria-Hungary's ally and agreed to assist Austria-Hungary in the invasion of Serbia. Russia, fearing an invasion of Serbia would end it's own influence in the Balkans began to mobilize for war. As a result, Germany declared war on Russia.
Austria declared war on Serbia because Serbian Gavrilo Princip had assassinated the archduke Franz Ferdinand and his wife in Austria in 1914. The war was declared on Serbia. Russia responded by aiding Serbia. Then Germany sent ultimatum to Russia demanding Russia to stop mobilizing and once the ultimatum was rejected by the Russians, Germany declared war on Russia.