Used adverbially to mean throughout the entire day, from morning to night. Often appears in literary or somewhat formal contexts.
I spent the whole day absorbed in reading.
last day of the month; last day of the year; New Year's Eve
Refers specifically to the final day of a month or year. When referring to the last day of the year, it can mean New Year's Eve. This usage is somewhat old-fashioned and may be encountered in traditional or literary contexts.
The last day of December is also called ōmisoka.
A Sino-Japanese borrowing of Chinese 尽日, used both for an entire day and for a day that exhausts or concludes a period, such as the last day of a month or year.