1) Psalms 69:14. 2) King David was pointing out that, in general, when people celebrate, they get boisterous and drunk and ''forget'' about the Creator; the Jews, however, when the time for the afternoon prayers comes on Shabbat, take a break from their eating, drinking and celebrating, and go to Synagogue to hear the Torah and pray to their Creator. 3) According to the Bach (O.H 292), the reason we say this verse describing an ''Eit Ratzon'' (opportune time) before the public Torah reading, is because the second set of tablets containing the Ten Commandments was given to the Jews on Yom Kippur, the day Moses finally came down from Mt. Sinai and began teaching the Torah. On that same day, the Jews were forgiven for building the Golden Calf and thus Yom Kippur forever became a day for forgiveness and atonement. Thus we see that the Torah is associated with ''opportune times'', times when G-d is open to our prayers. 4) According to the Zohar, the time of the afternoon prayers on Shabbat is an especially opportune time for prayers - which is not the case on a festival (see Aruch HaShulchan 292:1). This explains why this verse which mentions the words ''opportune time'' is said during the afternoon prayers on Shabbat even in circumstances where there is no Torah scroll, and even when an individual is praying alone at home. It also explains why it is not said during the afternoon prayers on a festival.

1) Psalms 119:142, 71:19 and 36:7. 2) When a dead person is brought to the cemetery, the mourners say a prayer called ''Tziduk HaDin'' (Acceptance of Judgement), through which they declare their acceptance of Divine judgement, and request mercy for the living. 3) At the time for the afternoon prayers on Shabbat. 4) Tachanun is a supplication said immediately after the Shemoneh Esreh of the morning and afternoon prayers on weekdays. Some examples of times when Tachanun is not said are: Shabbat, Festivals, Rosh Hodesh, and at the afternoon prayers preceding those days. 5) Tachanun, which is a supplication for mercy despite our sins, is not said in a mourner's home, because we don't want to do anything that might arouse Divine judgement in a house that has just experienced it (Aruch HaShulchan O.H 131:14).

לאחר החזרת השמונה עשרה נוהגין לומר צדקתך צדק והמה שלשה פסוקים כעין צדוק הדין על שלשה צדיקים שנפטרו בשעה זאת יוסף משה ודוד ואם הוא ביום שאם היה חול לא היו אומרים תחנון אין אומרים אותו אך כשמתפללין בצבור בבית אבל אומרים אותו דאם לא יאמרוהו הוי פרהסיא ואין אבילות פרהסיא בשבת

1) The first three blessings and the last three blessings of the weekday Shemoneh Esreh, are also said during the Shabbat and Festival Shemoneh Esreh prayers. However, on weekdays, the middle section of the Shemoneh Esreh consists of thirteen separate blessings, whereas the Shabbat and Festival Shemoneh Esreh prayers only have one short blessing in the middle section. 2) The blessing beginning with the word ''Be pleased'' is the first of the final three blessings of both the weekday and the Shabbat and Festival Shemoneh Esreh prayers. 3) Since the Sages could have just changed the one, common blessing and left the rest the same, if we start saying one of these - what could have been - common blessings, it is not considered as a real error.