PETIḤAH (Heb. פְּתִיחָה; "opening"), the ritual of opening the Ark in the synagogue during services to take out the Torah scroll(s) for the reading of the Law, and (particularly in Ashkenazi synagogues) to recite prayers of special importance or solemnity, especially on the High Holidays (e.g., the prayer *Avinu Malkenu and the entire *Ne'ilah service on the Day of Atonement). In the Reform ritual other special prayers (e.g., for the welfare of the government) are also recited before the open Ark. The custom of the petiḥah may be a remnant of the ritual in the talmudic period when in times of danger and need (pestilence, drought), the Ark was carried to the town square where special penitential prayers were recited (see Ta'an 2:1, 2, etc.). Mordecai Jaffe (in his Levush Tekhelet to Sh. Ar., oḤ 133) explains the custom of the petiḥah: "The high priest entered the Holy of Holies in the Temple once a year, on the Day of Atonement, in order to stress the special sanctity of that day; therefore the most significant prayers are recited before the open Ark to stress their special importance." The congregants rise for all prayers which are recited when the Ark is open.
"Petiḥah." Encyclopaedia Judaica. . Encyclopedia.com. (October 16, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/petihah
"Petiḥah." Encyclopaedia Judaica. . Retrieved October 16, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/petihah