The line between one year and the next. Stand right on the line and you get the party; step over it and look back at the weeks just after it, and you get the time when new things are supposed to start.
New Year 可以想成一年和下一年之間的那條線。站在線上的那一刻，是煙火、倒數和「新年快樂」。往前跨過去、回頭看剛開始的那幾個星期，就是「年後」——事情要重新排、計畫要重新開始的那段時間。同一個詞，差別只在鏡頭對準那一刻，還是對準那一段。
From the word’s history
The source simply gives new + year. The two everyday uses fall straight out of that: the moment the year turns, and the year that has just turned.
來源資料只說它是 new（新的）加 year（年）。兩個日常用法其實就從字面長出來：一個是「年剛翻過去的那一刻」，一個是「剛翻過去的那一年」。
Wiktionary entries for New Year / new year, packet evidence kaikki:3f6f69a190036ae4:482539305 ("From new + year.") and kaikki:3f6f69a190036ae4:1586709089.