Omisoka is on December 31, which is the last day of the year. It is an important day in the Japanese tradition, because it is the day before the new year's day, which is the most important holiday throughout the year.
Usually, people spend the day busily because there are lots of preparations to do for the next day. Also, it is common to clean the whole house on this day, in order to see the new year fresh.
At the very end of the day, usually around 11 o'clock at night, it is common to have "toshikoshi-soba," which is a kind of Japanese noodle. It is a tradition to eat soba to see the old year out and the new year.
There are many things to do on Omisoka, and it is a busy day for Japanese people. But it is an important day to end the year and to prepare to meet the new year.