Why doesn’t everyone say Le’dovid after Shacharit Mincha and Maariv? (I’ve heard that some do Shacharit and Mincha, and some do Shacharit and Maariv)
In the midrash (Vayikra Rabba 21) Chazal say that "Ori" refers to Rosh Hashana and "Yishi" refers to Yom Kippur. The source for saying Le’david Hashem Ori stems from different Mekubalim who wrote in their commentaries to the siddur that this mizmor in Tehillim has great spiritual value and that by saying it can annul bad decrees which descend from Heaven. (See Avodat hakodesh of the Chid"a) Indeed, the Chid"a suggested saying it 3 times a day after Shachrit, Mincha and Maariv especially during the month of Elul, but also the rest of the year. Others said to say it day and night. In the siddur "Tzlota d'avraham" the author comments that since no strictly defined times were set, each community chose their own minhag. Therefore, we find minhag ashkenaz which says Le’david Ma'ariv and Shacharit. Minhag sfard, Shacharit and Mincha. Among Edot Hamizrach , there are those who say Le'david after Shacharit the entire year and before Ma'ariv during the month of Elul. The minhag of the Vilna Gaon is to refrain from saying it.