This is the 8709th most frequent Japanese word.
春分
Vernal Equinox.
In this sentence, '春分' (shunbun) is used as part of '春分の日' (Shunbun no hi), referring specifically to the day around the vernal equinox celebrated as a holiday in Japan.
春分の日は日本の祝日の一つです。
The Vernal Equinox Day is one of Japan's public holidays.
Here, '春分' is used by itself to denote the astronomical event of the vernal equinox.
春分は昼と夜の長さがほぼ同じになる日です。
The vernal equinox is the day when the lengths of day and night are approximately equal.
'春分' in this sentence refers to the general time period of the vernal equinox, indicating a seasonal context.