During the Aseres Yemei Teshuva, the tefilla of Avinu Malkeinu is recited twice daily--once at Shacharis and once at Mincha. It is omitted on Friday at Mincha, on Shabbos and on Erev Yom Kippur (unless Yom Kippur falls on Shabbos). The custom in some congregations is to open the Aron Hakodesh during Avinu Malkeinu. Those who have this custom also stand during Avinu Malkeinu out of respect for the Sifrei Torah. During the verse of Avinu Malkeinu chotonu lefonecha, there are those who strike their chests when mentioning chotonu. Some Achronim discourage this practice, since the tefilla as a whole is not a form of confession.
[שו"ע תקפד, א, משנ"ב ד, ושעה"צ ז; ביאורים ומוספים דרשו, 4-3 ;שו"ע תרד, ב]