when nisioisin killed araragi on march 15th he was referencing the lamb sacrifice on the ides of march

"The Ides of each month were sacred to Jupiter, the Romans' supreme deity. The Flamen Dialis, Jupiter's high priest, led the "Ides sheep" (ovis Idulis) in procession along the Via Sacra to the arx, where it was sacrificed.[4]"

"A week later, on 22 March, the solemn commemoration of Arbor intrat ("The Tree enters") commemorated the death of Attis under a pine tree. A college of priests, the dendrophoroi ("tree bearers") annually cut down a tree,[14] hung from it an image of Attis,[15] and carried it to the temple of the Magna Mater with lamentations. The day was formalized as part of the official Roman calendar under Claudius (d. 54 AD).[16] A three-day period of mourning followed,[17] culminating with celebrating the rebirth of Attis on 25 March, the date of the vernal equinox on the Julian calendar.[18]
"
consistent with the tree associations for araragi and with his rebirth though the time period differs

wonder if he learned about it from those german philosopher books