In general, the year 1912 was a pretty bad year for the Ottomans. We’ve already talked about that year’s Albanian Revolt, which ended with Albania being granted substantial autonomy within the empire. The 1911-1912 Italo-Turkish War ended in October of that year with a victorious Italy in control of Libya and the Dodecanese islands (which now belong to Greece). But as that war was ending, a new one was starting (on October 8): the First Balkan War, fought between the Ottomans and the Balkan League (Serbia, Montenegro, Greece, and Bulgaria), which ended with the Ottomans losing nearly all of their European territory. As wars go, the cause of the First Balkan War was pretty simple: the Ottomans were obviously reeling, and those four Balkan states all wanted to gobble up as much of its European territory as they could. Yes, they formed a “league” to coordinate their war efforts, but each of the four had its own territorial ambitions, and they were all largely in conflict with one another (everybody was looking to annex Macedonia for themselves, for example). Those competing territorial claims made the immediate post-war settlement untenable, and led quickly to a Second Balkan War in mid-1913, with Bulgaria fighting its former allies (plus the Ottomans).
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.