The following attempts to explain why people pray at the grave of Tzadiks, and how the merit of the tzadik can work in favor of the one praying. My rabbi is traveling to New York in 2 weeks to attend the Rabbis Yartzeit at the Ohel...
http://ascentofsafed.com/cgi-bin/ascent.cgi?Name=b13q16Q:"How can you take people to pray at the graves of departed righteous Jews? We are supposed to pray only to G-d and to believe that G-d is the source of all blessings. Is it not "idol-worship" to pray to a tzadik at his grave?"A:Praying at "holy gravesites" is a time-honored and widespread Jewish practice. Of course, praying to the dead or even asking them for information, holding seances, etc. is clearly prohibited (see Deut.18:11) and is related to avodah zarah - idol worship. Therefore, we are obligated to assume that this is not what Jews at gravesites are doing. Outward appearances, and perhaps even the words of our sages in this matter, may be easy to misconstrue. Nevertheless, we must be very, very careful about labelling the practices of large numbers of G-d fearing Jews with the grievous sin of "idol-worship."
In discussing the verse [Num. 13:22], "They went up through the south and [he] came until Hebron...", the Talmud [Sota 34b] interprets it to imply that Kalev, the only one of the twelve spies besides Yehoshua who did not slander the land of Israel, came to Hebron alone [see also Joshua 14:6-15]. Why did Kalev make this solo side-trip? "He went to prostrate himself upon the graves of the Patriarchs. He said:"Fathers of the world, pray for me that I be saved [i.e. succeed in resisting -- Rashi on the verse] the evil counsel of the other spies."
Nobody thinks that Kalev prayed to Avraham, Yitzchak or Yaakov. Rather, he requested that they add their prayers to his own supplications that Heaven grant him the inner strength to follow through on his good intentions. He prayed specifically at their burial site in order that their merit combined with the holiness of their final resting place help make his prayers more acceptable.
The holiness of these gravesites is derived from the lingering connection of the soul to its point of final departure from the body after burial. Many of the previously unknown burial sites of famous tzadikim in the vicinity of Zefat were first identified by the Holy Ari [Rabbi Yitzchak Luria--1534-1572], who was able to sense the presence of the specific soul hovering about each place. This presence is especially strong on the tzadik's yahrzeit [anniversary of the passing].
The Zohar [III:70B ff--quoted in Tanya IV: 27 (p.292)] states that without the prayers of tzadikim, the world would not endure for a single moment. Tzadikim shield the world, and even more so after their death than in their earthly lives. A striking example is the Matriarch Rachel, who, we are told [Midrash Rabbah; Rashi and Ramban on Gen.49:7; haftorah, 2nd day Rosh HaShanah] was buried on the highway at Beth Lehem so that her descendants in exile after the destruction of the First Temple could pray at her grave and she could pray for them. Also, the Midrash movingly depicts [c.f. Me'am Lo'ez on Gen. 37:36] a distraught Yosef tearing himself away from his captors and weeping hot tears over his mother's grave.
In our days, we too, utilize this same principle of calling upon the merit of departed tzaddikim, their benevolent prayers, and the holiness of their final resting places. In fact, not only is this practice permitted, it is recommended; and in certain situations, such as severe droughts, it is actually legislated [Taanit 16a].
Moreover, the model of Kalev praying at the burial site of the Patriarchs is extended not only to the resting places of tzadikim but also to one's personal forebearers. One of the classic halachic commentators, the Bach [Rabbi Joel Sirkes 1561-1640, in Yorah Deah end of 217], strongly approved the practice of praying at one's ancestors' graves in times of difficulty, since their merit can intervene to help avert an unpleasant decree. Indeed, it is an almost universal Jewish custom to visit the graves of close relatives on the anniversary of their passing and to pray there.
This practice is cited in Jewish Law in connection with the High Holidays.
After prayers on the morning preceding Rosh Hashanah, it is customary to go to the cemetery and there to pray and give charity to the poor. Since a cemetery is the place where the righteous rest, prayers are more readily acceptable there. Circle the grave and give charity before saying the supplications.
Shulchan Aruch I, 581:4 with Mishna Brurah 27]
And when you do this, remember!
Supplications are made to exhort the tzaddikim there to intercede for us on the day of judgement. However, we do not direct our prayers toward the dead who rest there; rather, we implore G-d to have mercy on us for their sake. [Kitzur Shulchan Aruch 128:13]
May our prayers find favor in G-d's eyes. May we all be inscribed for a good and sweet year.
Sincerely,
Yrachmiel Tilles