noun
month-end soba; year-end soba
Soba noodles traditionally eaten on the last day of the month, especially on New Year's Eve (大晦日).
大晦日には晦日蕎麦を食べる習慣がある。
There is a custom of eating year-end soba on New Year's Eve.
晦日蕎麦は月末の縁起物として親しまれている。
Month-end soba is enjoyed as a good-luck charm at the end of the month.
年越し蕎麦 specifically refers to soba eaten on New Year's Eve, while 晦日蕎麦 can refer to soba eaten on the last day of any month.
From 晦日 (misoka, 'last day of the month') + 蕎麦 (soba, 'buckwheat noodles'). The exact origin of the custom is uncertain, but it is associated with wishes for longevity and financial prosperity.