Horse New Year's Eve - Umanoto Toshikoshi

...It refers to the period from the evening of January 6th to the 7th. It is also called "Kami no toshigoshi" (God's New Year's Eve) or "Uma no toshigoshi" (Horse's New Year's Eve). January 7th can be considered the end of New Year's Day or the beginning of the 15th day of the New Year, but in the Edo period it was considered an important festival as one of the five seasonal festivals (Jinjitsu), and perhaps for that reason it is also called Nanaka Shogatsu (Seventh New Year's Day).

*Some of the terminology that refers to "Horse New Year's Eve" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…1月6日夕方から7日にかけてをいう。〈神年越し〉〈馬の年越し〉などともいう所もある。1月7日は元日正月の終りだとも十五日正月の始まりだとも考えられるが，また，江戸時代には五節供の一つ(人日（じんじつ）)として重要な節日と考えられており，そのためか七日正月とも呼ばれている。…

※「馬の年越し」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報