'Great last day of the month'. The 31 st of December, final day of the old year. It precedes ganjitsu (New Year's day) as part of the shogatsu (New Year) season, one of the most important periods in the Japanese ritual calendar. Home shrines (both kamidana and butsudan, Buddhist ancestral altars) are cleaned ready for the kami and the ancestors of new year. Omisoka is generally marked by the ringing of a Buddhist temple bell (joya no kane, the watch-night bell) 108 times to drive out all the sins of the old year. It is an integral part of the 'demons out, good luck in' motif of the Japanese new year which transcends religious distinctions, continues with hatsumode (the first shrine visit) the following day and is reiterated at the lunar new year with setsubun.
A Popular Dictionary of Shinto. Brian Bocking.