Short answers, pulled from the story.
The Equus October sacrifice occurred on the Ides of October, a specific date within the Roman calendar. Ancient sources indicate this event happened during one of three festivals honoring Mars alongside the Equirria on February 27 and March 14.
The ritual took place within the Campus Martius, an alluvial plain along the Tiber river that stood outside the pomerium. This field served as both horse pasturage and a drill field for youth performing exercises in arms according to Dionysius of Halicarnus.
Two-horse chariot races known as bigae were held to determine the victim before the right-hand horse was transfixed by a spear. Festus described how the head and tail were cut off for separate ceremonial uses with blood carried quickly enough to drip onto the sacred hearth of the Regia.
A Roman farmer also functioned as a soldier during the Republic era creating a unique convergence of roles where Verrius Flaccus explained a horse is suited for war just as an ox is suited for tilling crops. The sacrifice took place ob frugum eventum meaning either in thanks for the completed harvest or for the sake of the next one after winter wheat was sown in the fall.
The head of the sacrificed horse carried significant talismanic potency within Roman belief systems while bread pendants were attached though their exact shape remains unrecorded. The tail held its own distinct power often interpreted as a symbol of fertility or phallic potency according to George Devereux who argued cauda might serve as a euphemism for the penis.
Most scholars identify Etruscan influence on the early formation of Roman ceremonies involving horses since the Campus Martius was formerly known as ager Tarquiniorum land belonging to the Etruscan king Tarquinius Superbus. Timaeus attempted to link the sacrifice to the Trojan Horse claiming Romans commemorated Troy's fall by spearing a war-horse while Polybius dismissed this explanation as childish.