Because it's offered as shogon (to decorate Buddha statues and temples) at shusho-e (the New Year's service), kagami-mochi is displayed after the devotional exercises on the morning of December 31st and before the devotional exercises in the afternoon.
正月（修正会）の荘厳として供える為、12月31日の朝の勤行のあと、夕の勤行までに飾る。