This is what I have been able to find about Autumnal Equinox in Hindu/Aryan literature. It was known as the Vishuvan day:
"The central day of the Sattra is called Vishuvân, and stands by itself, dividing the Sattra into two equal halves like the wings of a house (Tait. Br. I, 2, 3, 1); and the rites in the latter half of the session or after the Vishuvân day are performed in an order which is the reverse of that followed in forming the ceremonies in the first half of the sacrifice. The model annual Sattra (Indo-Aryan ritual cycle - the Gavâm-anayam - cow's walk) thus, consists of the following parts:
Days Parts
1 The introductory Atirâtra
1 The Chaturvimsha day, otherwise called the Ârambhaniya (Aît. Br. IV, 12), or the Prâya nîya (Tând. Br. IV. 2 the real beginning of the Sattra
150 Four Abhiplava, followed by one Prishthya shalaha each month; continued in this way for five months
24 Three Abhiplava and one Prishthya shalaha
1 The Abhijit day
3 The Three Svara-Sâman days
Vishuvân or the Central day which stands by itself i.e., not counted in the total of the Sattra days
3 The three Svara-Sâman days
1 The Vishvajit day
24 One Prishthya and three Abhiplava shalahas
120 One Prishthya and four Abhiplava shalahas each month continued in this way for four months
30 Three Abhiplava shalahas, one Go-shtoma, one Âyu-shtoma, and one Dasharâtra (the ten days of Dvâdashâha), making up one month
1 The Mahâvrata day, corresponding to the Chaturvimsha day at the beginning
1 The concluding Atirâtra
360 Total days
It will be seen from the above scheme that there are really a few sacrificial rites which are absolutely fixed and unchangeable in the yearly Sattra. The two Atirâtras, the introductory and the concluding, the Chaturvimsha and the Mahâvrata day, the Abhijit and the Vishvajit, the three Svara-Sâman days on either side of Vishuvân, the Vishuvân itself, and the ten days of Dvâdashâha, making up 22 days in all exclusive of Vishuvân, are the only parts that have any specialty about them. The rest of the days are all made up by Abhiplava and Prishthya shalahas which therefore constitute what may be called the elastic or the variable part of the yearly Sattra."
Vishuvan was counted among the 12 days of Calends, the Dvadashaha.
Source: 'Arctic Home in Vedas', Bal Gangadhar Tilak
