The year 1912 does not stand in the history books as a particularly good one for the Ottoman Empire. There was the January-August Albanian Revolt, which ended with the Ottomans forced to grant substantial autonomy and other concessions to their restive Albanian minority. There was the Italo-Turkish War, which began in 1911 and ended, badly for the Ottomans, in October 1912. The empire lost Libya and the Dodecanese Islands in that conflict, which among other things featured the first use of airplanes in combat. And then there was the First Balkan War, fought between the Ottomans and the Balkan League (Serbia, Montenegro, Greece, and Bulgaria), which began on October 8, 1912. It ended the following May with the Ottomans losing nearly all of their European territory.
At its essence, the cause of the First Balkan War was pretty simple: the Italo-Turkish War and the Albanian Revolt showed that the Ottoman Empire was reeling, and those four Balkan states each saw an opportunity for expansion at Ottoman expense. While they coordinated their war efforts via their ad hoc league, each member had its own territorial ambitions, which in some cases were in direct conflict with one another. Serbia wanted Kosovo, Bulgaria wanted Thrace, Greece wanted Crete and the parts of Thessaly that it had lost in the Greco-Ottoman War of 1897, and everybody wanted Balkan Macedonia. But despite their separate ambitions, the decision to unify their war efforts enabled them to divide the Ottoman army and overwhelm it along multiple fronts.
Although October 24 was still early in the war (it wouldn’t end until May 1913), it’s also distinguished by the fact that the Ottomans suffered two separate defeats on the same day. Neither one of the defeats was decisive or particularly surprising, and I’m only (briefly) mentioning them at all because they both happened on the same day and highlighted the fact that the Ottomans were seriously overmatched by their former Balkan subjects by this point. First was the Battle of Kirk Kilisse (or Lozengrad), fought in eastern Thrace between Ottoman and Bulgarian armies. The vastly outnumbered Ottomans (about 95,000 men to Bulgaria’s ~150,000) simply couldn’t withstand the Bulgarian Third Army’s attack and crumbled.
The second defeat came at the Battle of Kumanovo, which is now in Macedonia, at the hands of the Serbs. Again the Ottomans (under Field Marshal Zeki Pasha) were badly outnumbered (this is really a sentence you don’t see much until the 19th-20th centuries), with about 60,000 men against 130,000 Serbs (with one Bulgarian division), and they no longer had any sort of technological advantage in firepower either. The forces were so mismatched that a heavy Ottoman attack on the left wing of the Serbian line on October 23 failed despite the fact that the main command of the whole Serbian army (under General Radomir Putnik) seems to have been unaware that the attack was happening (or was at least unaware of how heavy the fighting was) and never sent any reserve units in to reinforce its left flank. The next day, when the full Serbian army was engaged, the Ottomans were pretty easily defeated and their forces retreated. It was a crushing Serbian victory, but in truth Zeki Pasha actually lucked out; his decision to attack a far superior force on October 23 was probably not the smartest thing he could have done, and the only thing that kept his forces from being routed on that day was probably that miscommunication among the Serbs.
As I said above, neither of these battles was incredibly decisive, but Kumanovo especially did help to set the tone that the rest of the war would follow (the Balkan League on offense, the Ottomans trying and mostly failing to fend them off). Kirk Kilisse, meanwhile, opened the way up for the Bulgarian Third Army to join the Bulgarian First Army and push toward Constantinople. They were met by another Ottoman army at the First Battle of Çatalca (on the outskirts of modern Istanbul) in November, and this time the Ottomans actually won.